Replace stub federation handlers with full implementations that accept
relay and proxy requests from peer servers. Adds federation_client and
local_domain fields to ServerState for outbound relay and federated
address resolution. All six handlers (relay_enqueue, relay_batch_enqueue,
proxy_fetch_key_package, proxy_fetch_hybrid_key, proxy_resolve_user,
federation_health) now validate federation auth, interact with local
storage, and wake waiters on message delivery.
Christian Nennemann
1d59a052ad