INT-3551: Idempotent Receiver: Add value-strategy

JIRA: https://jira.spring.io/browse/INT-3551

* Rename `MetadataKeyStrategy` -> `MetadataEntryStrategy`
* Add `valueStrategy` to the `MetadataStoreSelector`
* Add `value-strategy` and `value-expression` to the `<idempotent-receiver>`

INT-3551: Add `@IR` support on service methods

Rework `MetadataKeyStrategy` just to the `MessageProcessor`
Fix Docs

Minor Doc Polishing.