cmd/strelaysrv: Add optional auth token (fixes #3987) (#8561)

* implement authentication via token for relaysrv

Make replaysrv check for a token before allowing clients to
join. The token can be set via the replay-uri.

* fix formatting

* key composite literal

* do not error out if auth material is provided but not needed

* remove unused method receiver

* clean up unused parameter in functions

* cleaner token handling, disable joining the pool if token is set.

* Keep backwards compatibility with older clients.

In prior versions of the protocol JoinRelayRequest did not have a
token field. Trying to unmarshal such a request will result in
an error. Return an empty JoinRelayRequest, that is a request
without token, instead.

Co-authored-by: entity0xfe <entity0xfe@my.domain>

lib/relay/protocol/packets.go

@@ -31,9 +31,12 @@ type header struct {
 
 type Ping struct{}
 type Pong struct{}
-type JoinRelayRequest struct{}
 type RelayFull struct{}
 
+type JoinRelayRequest struct {
+	Token string
+}
+
 type JoinSessionRequest struct {
 	Key []byte // max:32
 }