INT-2426: Add `Idempotent Receiver` EIP pattern

JIRA: https://jira.spring.io/browse/INT-2426

**DO NOT MERGE YET**

It's just an initial commit, like a PoC to review before further work.
This won't scale with large groups (getting all messages), especially with a persistent message store.

We really need a `MGS` where we can query by group + message key.