[client] Extract Handshaker and Signaler into a signaling package

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.

client/internal/peer/event.go

@@ -5,6 +5,7 @@ import (
 
 	"github.com/pion/ice/v4"
 
+	"github.com/netbirdio/netbird/client/internal/peer/signaling"
 	"github.com/netbirdio/netbird/route"
 )
 
@@ -21,11 +22,11 @@ type evClose struct {
 }
 
 type evRemoteOffer struct {
-	offer OfferAnswer
+	offer signaling.OfferAnswer
 }
 
 type evRemoteAnswer struct {
-	answer OfferAnswer
+	answer signaling.OfferAnswer
 }
 
 type evRemoteCandidate struct {