INT-3381: Messaging Annotations on `@Bean` Method

JIRA: https://jira.spring.io/browse/INT-3381

* Add support to mark `@Bean` methods with `Messaging Annotations`
* Addition attributes for `Messaging Annotations`
* Some refactoring and fixing
* Fix `AbstractMappingMessageRouter#onInit()` propagation
* Fix `GemfireGroupStoreTests` (https://build.spring.io/browse/INT-MASTERSPRING40-JOB1-246/test/case/135235772)

There is still some side effect: we need define `MessageChannel` beans explicitly. Since methods are processed within `BPP`,
not all beans with `Messaging Annotations` might be processed.

INT-3381: PR comments and other fixes

INT-3381 Polishing

INT-3381: Revert `RouterFB` & `SplitterFB`
INT-3375: Docs for Messaging Annotations

JIRA: https://jira.spring.io/browse/INT-3375

INT-3375 Doc Polishing