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.
Reason?
Similar attrs have singular names too: e.g. `pattern` on the `<channel-interceptor>` or `path` on the `<http:inbound-gateway>`