metadata-2 (#2273)

Initial implementation of draft/metadata-2

irc/client.go

@@ -131,6 +131,7 @@ type Client struct {
 	clearablePushMessages   map[string]time.Time
 	pushSubscriptionsExist  atomic.Uint32 // this is a cache on len(pushSubscriptions) != 0
 	pushQueue               pushQueue
+	metadata                map[string]string
 }
 
 type saslStatus struct {
@@ -214,6 +215,8 @@ type Session struct {
 	batch MultilineBatch
 
 	webPushEndpoint string // goroutine-local: web push endpoint registered by the current session
+
+	metadataSubscriptions utils.HashSet[string]
 }
 
 // MultilineBatch tracks the state of a client-to-server multiline batch.
@@ -1129,6 +1132,7 @@ func (client *Client) SetNick(nick, nickCasefolded, skeleton string) (success bo
 	client.nickCasefolded = nickCasefolded
 	client.skeleton = skeleton
 	client.updateNickMaskNoMutex()
+
 	return true
 }