Introduce a post_registration/0 callback for peer discovery backends

Meant to be used for tasks that should be done after we've successfully registered,
e.g. setting up periodic operations such as health checks.

Author: michaelklishin

src/rabbit_peer_discovery_backend.erl

@@ -43,3 +43,5 @@
 -callback register()   -> ok | {error, Reason :: string()}.
 
 -callback unregister() -> ok | {error, Reason :: string()}.
+
+-callback post_registration()   -> ok | {error, Reason :: string()}.