GH-441: Add JsonBytesToMap function (#461)

Add `JsonBytesToMap` function to allow conversion between byte[] and JSON Map.

Fixes https://github.com/spring-cloud/stream-applications/issues/441

* Introduce a `JsonBytesToMap` as a part of a `payload-converter-function` module
which is auto-discovered by Spring Cloud Function scanning algorithm - the `functions` package.
* Add a `payload-converter-function` as dependency into an `aggregator-function`
* Compose `jsonBytesToMap|aggregatorFunction` for the `aggregator-processor`
* Verify a `JsonBytesToMap` function in action with an `AggregatorProcessorTests`
* Mentioned such a payload conversion in the `aggregator-processor` README

function/aggregator-function/pom.xml

@@ -15,6 +15,11 @@
     <description>Spring Native Function for Aggregator</description>
 
     <dependencies>
+        <dependency>
+            <groupId>org.springframework.cloud.fn</groupId>
+            <artifactId>payload-converter-function</artifactId>
+            <version>${project.version}</version>
+        </dependency>
         <dependency>
             <groupId>org.springframework.integration</groupId>
             <artifactId>spring-integration-core</artifactId>