GH-107: Make Splitter Function as Flux-based

Fixes: https://github.com/spring-cloud/spring-functions-catalog/issues/107

When we have a composition like this:

```
spring.cloud.function.definition = fileSupplier|splitterFunction
```

Then final "function" signature is like this `Supplier<Flux<Message<List<Message<?>>>>>`.
And that is exactly what we don't expected from the splitter in the end of the composition.
While Spring Cloud Stream supports de-batching, it works for a `List` output only if function is bound by itself.
In case of composition we got just a `Supplier`.

* Rework `SplitterFunctionConfiguration` for `splitterFunction` from `Function<Message<?>, List<Message<?>>>`
to `Function<Flux<Message<?>>, Flux<Message<?>>>` signature to support every possible simple and composed bindings
in Spring Cloud Stream
* Rework `SplitterFunctionApplicationTests` for new expected `Function<Flux<Message<?>>, Flux<Message<?>>>` signature
* Rework `zip-split-rabbit-binder` sample to not use a `flattenFunction` workaround
and fully rely on whatever is new for the `splitterFunction`
* Fix `ZipSplitRabbitBinderApplicationTests` moving the `@RabbitListener` into a `@TestConfiguration`.
Apparently in a new Spring Boot version the test class is registered as a bean much later than normal application context startup.
Therefore, even if the `@RabbitListener` parsed and registered properly, the `RabbitAdmin` bean
has been already started to see our extra bean definition for the `@QueueBinding`

Changing signature for the splitterFunction to reactive types would make it working even with a Supplier composition.

Fix JDBC & MongoDB suppliers to deal with a new version of Splitter function

Fix Checkstyle violations

Use `IntegrationReactiveUtils.messageSourceToFlux()` API

The `IntegrationReactiveUtils.messageSourceToFlux()` provides convenient API to represent a `MessageSource`
as a `Flux` to poll this source.
The API has an error handling logic and delay when no data emitted by the source

* Remove `org.springframework.cloud` dependencies from the project
since we don't use `@PollableBean` anymore, which comes from the `spring-cloud-function-context`
* Simplify `JdbcSupplierConfiguration` and `MongodbSupplierConfiguration` code more: more injections to the respective bean method.
* Use `(__) ->` lambda syntax for unused argument
* Remove unused `ThreadLocalFluxSinkMessageChannel` internal class
* Update Copyrights of the classes in this change

Upgrade to Gradle `8.12`

samples/zip-split-rabbit-binder/README.adoc

@@ -18,8 +18,7 @@ The second one is for `UnZipTransformer`, which we use for a custom function to
 The `splitterFunction` is used in a `FileSplitter` mode to read lines from unzipped entries and emit each of them as an individual message.
 Essentially, we are splitting twice: zip entries, and content of each file.
 
-The composition is like this: `fileSupplier|unzipFunction|splitterFunction|flattenFunction`.
-(The `flattenFunction` will be explained latter).
+The composition is like this: `fileSupplier|unzipFunction|splitterFunction.
 The result of this composition is a `Supplier<Flux<Mesage<?>>>` and we bind it into a RabbitMQ `unzipped_data_exchange` using Spring Cloud Stream.
 
 For `fileSupplier` we provide these configuration properties:
@@ -49,8 +48,6 @@ Which is a trigger for that function to use a `FileSplitter` for zip entries to
 The custom `ZipSplitRabbitBinderApplication.unzipFunction()` (might be a candidate for the future Functions Catalog version) uses `Flux` API to unzip polled files via `UnZipTransformer` and then `flatMapIterable()` for zip entries.
 Then those entries are fed into a `splitterFunction` for `FileSplitter` mode.
 
-The mention `ZipSplitRabbitBinderApplication.flattenFunction()` is needed for now here since `splitterFucntion` produces a `List<Message>` which cannot be https://docs.spring.io/spring-cloud-stream/reference/spring-cloud-stream/producing-and-consuming-messages.html#batch-producers[de-batched] by Spring Cloud Stream since our final product of the composition is, essentially, `Supplier<Flux<Message<?>>>`.
-
 To run the application from main `ZipSplitRabbitBinderApplication` class (`./gradlew bootRun`), the RabbitMQ broker must be supplied on the target environment.
 
 The test environment for this sample uses `org.springframework.boot:spring-boot-testcontainers` and `org.testcontainers:rabbitmq` to run RabbitMQ in Docker container and wire it properly into Spring Boot auto-configuration.

samples/zip-split-rabbit-binder/src/main/java/com/example/ZipSplitRabbitBinderApplication.java

@@ -1,7 +1,6 @@
 package com.example;
 
 import java.io.File;
-import java.util.List;
 import java.util.Map;
 import java.util.function.Function;
 
@@ -34,10 +33,4 @@ public class ZipSplitRabbitBinderApplication {
 			.flatMapIterable(Map::values);
 	}
 
-	// TODO until 'splitterFunction' is fixed this way: https://github.com/spring-cloud/spring-functions-catalog/issues/107
-	@Bean
-	Function<Flux<Message<List<Message<?>>>>, Flux<Message<?>>> flattenFunction() {
-		return messageFlux -> messageFlux.map(Message::getPayload).flatMapIterable(Function.identity());
-	}
-
 }

samples/zip-split-rabbit-binder/src/main/resources/application.yml

@@ -4,11 +4,11 @@ spring:
 
   cloud:
     function:
-      definition: fileSupplier|unzipFunction|splitterFunction|flattenFunction
+      definition: fileSupplier|unzipFunction|splitterFunction
 
     stream:
       bindings:
-        fileSupplier|unzipFunction|splitterFunction|flattenFunction-out-0:
+        fileSupplier|unzipFunction|splitterFunction-out-0:
           destination: unzipped_data_exchange
 
 file:

samples/zip-split-rabbit-binder/src/test/java/com/example/ZipSplitRabbitBinderApplicationTests.java

@@ -19,6 +19,7 @@ import org.springframework.amqp.rabbit.annotation.Queue;
 import org.springframework.amqp.rabbit.annotation.QueueBinding;
 import org.springframework.amqp.rabbit.annotation.RabbitListener;
 import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.boot.test.context.TestConfiguration;
 import org.springframework.boot.testcontainers.service.connection.ServiceConnection;
 import org.springframework.test.annotation.DirtiesContext;
 
@@ -47,11 +48,16 @@ class ZipSplitRabbitBinderApplicationTests {
 		}
 	}
 
-	@RabbitListener(bindings = @QueueBinding(value = @Queue,
-			exchange = @Exchange(value = "unzipped_data_exchange", type = ExchangeTypes.TOPIC), key = "#"))
-	void receiveDataFromSplittedZips(String payload) {
-		LOG.info("A line from zip entry: " + payload);
-		DATA_SINK.offer(payload);
+	@TestConfiguration
+	static class RabbitListenerTestConfiguration {
+
+		@RabbitListener(bindings = @QueueBinding(value = @Queue,
+				exchange = @Exchange(value = "unzipped_data_exchange", type = ExchangeTypes.TOPIC), key = "#"))
+		void receiveDataFromSplittedZips(String payload) {
+			LOG.info("A line from zip entry: " + payload);
+			DATA_SINK.offer(payload);
+		}
+
 	}
 
 }