GH-8626: Provide cleaner transform() DSL (#8653)

* GH-8626: Provide cleaner `transform()` DSL

Fixes https://github.com/spring-projects/spring-integration/issues/8626

* Add missed `transform(String beanName, @Nullable String methodName)` API
* Introduce a `TransformerSpec` to expose a strict API to configure
transformer variants.
* Introduce `transformWith(Consumer<TransformerSpec>)` as a single point
of all possible transformer and its endpoint options
* Deprecate those `IntegrationFlowDefinition.transform()` variants
which are harder to configure as several lambda arguments

This change will make Kotlin & Groovy DSL more readable and straightforward

* * Use new `transformWith()` in tests where a deprecated API still used
* Add JavaDocs to `TransformerSpec`
* Fix generic types for `BaseIntegrationFlowDefinition.transformWith()` -
they make sense exactly on the `TransformerSpec.transformer()` only
* Apply `transformWith()` for Groovy DSL
* Introduce a new `ClassUtils.isLambda(Object candidate)`
and add a check for Groovy `Closure`
* Fix `GroovyIntegrationFlowDefinition.createConfigurerIfAny()` to propagate a `Consumer` argument
down to the `Closure`

* * Rename `TransformerSpec -> TransformerEndpointSpec` for better context
meaning of the class
* Introduce `KotlinTransformerEndpointSpec` as an extension of the `TransformerEndpointSpec`
to have an `inline fun <reified P> transformer(crossinline function: (P) -> Any)`
for Kotlin style
* Add `KotlinIntegrationFlowDefinition.transformWith(KotlinTransformerEndpointSpec)`
* Deprecate Kotlin methods which are covered by the mentioned `transformWith()`
* Fix tests to use new API
* Mentioned the change in the doc

spring-integration-webflux/src/test/java/org/springframework/integration/webflux/observation/WebFluxObservationPropagationTests.java

@@ -212,7 +212,9 @@ public class WebFluxObservationPropagationTests {
 							.payloadExpression("#requestParams.name[0]")
 							.requestChannel(webFluxRequestChannel)
 							.id("webFluxGateway"))
-					.<String, String>transform(String::toLowerCase, e -> e.id("testTransformer"))
+					.transformWith(t -> t
+							.<String, String>transformer(String::toLowerCase)
+							.id("testTransformer"))
 					.get();
 		}