Fixes spring-projects#3902

* Add `isAsync()` propagation from the `MessagingMethodInvokerHelper`
to the `AbstractMessageProducingHandler` to set into its `async` property.
The logic is based on a `CompletableFuture`, `Publisher` or Kotlin `suspend`
return types of the POJO method
* Introduce `IntegrationMessageHandlerMethodFactory` and `IntegrationInvocableHandlerMethod`
to extend the logic to newly introduced `ContinuationHandlerMethodArgumentResolver`
and call for Kotlin suspend functions.
* Remove `MessageHandlerMethodFactoryCreatingFactoryBean` since its logic now is covered with the
`IntegrationMessageHandlerMethodFactory`
* Kotlin suspend functions are essentially reactive, so use `CoroutinesUtils.invokeSuspendingFunction()`
and existing logic in the `AbstractMessageProducingHandler` to deal with `Publisher` reply

* Fix `GroovySplitterTests` for the current code base

* Add `kotlinx.coroutines.flow.Flow` support
The `Flow` is essentially a multi-value reactive `Publisher`,
so use `ReactiveAdapterRegistry` to convert any custom reactive streams result to `Flux` and `Mono`
which we already support as reply types

* Add docs for `Kotlin Coroutines`
Rearrange the doc a bit extracting Kotlin support to individual `kotlin-functions.adoc` file

* Fix missed link to `reactive-streams.adoc` from the `index-single.adoc`
* Fix unintended Javadocs formatting in the `AbstractMessageProducingHandler`

* Add suspend functions support for Messaging Gateway
* Add convenient `CoroutinesUtils` for Coroutines types and `Continuation` argument fulfilling via `Mono`
* Treat `suspend fun` in the `GatewayProxyFactoryBean` as a `Mono` return
* Convert `Mono` to the `Continuation` resuming in the end of gateway call

* Document `suspend fun` for `@MessagingGateway`

* Mention default sync behavior in the docs

Co-authored-by: Gary Russell <[email protected]>