[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/store_test.go

@@ -0,0 +1,49 @@
+package store
+
+import (
+	"testing"
+
+	"github.com/netbirdio/netbird/relay/messages"
+)
+
+type MocPeer struct {
+	id messages.PeerID
+}
+
+func (m *MocPeer) Close() {
+
+}
+
+func (m *MocPeer) ID() messages.PeerID {
+	return m.id
+}
+
+func TestStore_DeletePeer(t *testing.T) {
+	s := NewStore()
+
+	pID := messages.HashID("peer_one")
+	p := &MocPeer{id: pID}
+	s.AddPeer(p)
+	s.DeletePeer(p)
+	if _, ok := s.Peer(pID); ok {
+		t.Errorf("peer was not deleted")
+	}
+}
+
+func TestStore_DeleteDeprecatedPeer(t *testing.T) {
+	s := NewStore()
+
+	pID1 := messages.HashID("peer_one")
+	pID2 := messages.HashID("peer_one")
+
+	p1 := &MocPeer{id: pID1}
+	p2 := &MocPeer{id: pID2}
+
+	s.AddPeer(p1)
+	s.AddPeer(p2)
+	s.DeletePeer(p1)
+
+	if _, ok := s.Peer(pID2); !ok {
+		t.Errorf("second peer was deleted")
+	}
+}