Move the signaling protocol out of the peer package into
client/internal/peer/signaling: OfferAnswer, IceCredentials, Handshaker
and Signaler. Break the peer<->signaling cycle by giving the Handshaker
a plain Config and an ICEWorker interface instead of *Conn/*WorkerICE,
and by passing the relay manager directly rather than the relay worker.
Combine the ICE worker's local-credentials and session-id accessors into
a single Credentials() returning a Credentials struct.
Move the ICE session id to the ice package as ice.SessionID, since it
identifies an ICE agent session and is minted there alongside
GenerateICECredentials; signaling only carries it.
917e85f648