[client, relay-server] Feature/relay notification (#4083)

- Clients now subscribe to peer status changes.
- The server manages and maintains these subscriptions.
- Replaced raw string peer IDs with a custom peer ID type for better type safety and clarity.

relay/server/store/notifier.go

@@ -0,0 +1,64 @@
+package store
+
+import (
+	"context"
+	"sync"
+
+	"github.com/netbirdio/netbird/relay/messages"
+)
+
+type PeerNotifier struct {
+	store *Store
+
+	listeners      map[*Listener]context.CancelFunc
+	listenersMutex sync.RWMutex
+}
+
+func NewPeerNotifier(store *Store) *PeerNotifier {
+	pn := &PeerNotifier{
+		store:     store,
+		listeners: make(map[*Listener]context.CancelFunc),
+	}
+	return pn
+}
+
+func (pn *PeerNotifier) NewListener(onPeersComeOnline, onPeersWentOffline func([]messages.PeerID)) *Listener {
+	ctx, cancel := context.WithCancel(context.Background())
+	listener := newListener(pn.store)
+	go listener.listenForEvents(ctx, onPeersComeOnline, onPeersWentOffline)
+
+	pn.listenersMutex.Lock()
+	pn.listeners[listener] = cancel
+	pn.listenersMutex.Unlock()
+	return listener
+}
+
+func (pn *PeerNotifier) RemoveListener(listener *Listener) {
+	pn.listenersMutex.Lock()
+	defer pn.listenersMutex.Unlock()
+
+	cancel, ok := pn.listeners[listener]
+	if !ok {
+		return
+	}
+	cancel()
+	delete(pn.listeners, listener)
+}
+
+func (pn *PeerNotifier) PeerWentOffline(peerID messages.PeerID) {
+	pn.listenersMutex.RLock()
+	defer pn.listenersMutex.RUnlock()
+
+	for listener := range pn.listeners {
+		listener.peerWentOffline(peerID)
+	}
+}
+
+func (pn *PeerNotifier) PeerCameOnline(peerID messages.PeerID) {
+	pn.listenersMutex.RLock()
+	defer pn.listenersMutex.RUnlock()
+
+	for listener := range pn.listeners {
+		listener.peerComeOnline(peerID)
+	}
+}