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.
INT-3557 Clarify MessageHeaders Constants

JIRA: https://jira.spring.io/browse/INT-3557

Add class names and code tags.

Add missing header constants.

Polishing for `timestamp` header doc.

Fix JavaDocs issues: https://build.spring.io/browse/INT-EIGHTAPI-296