[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/auth/validator.go

@@ -7,13 +7,6 @@ import (
 	authv2 "github.com/netbirdio/netbird/relay/auth/hmac/v2"
 )
 
-// Validator is an interface that defines the Validate method.
-type Validator interface {
-	Validate(any) error
-	// Deprecated: Use Validate instead.
-	ValidateHelloMsgType(any) error
-}
-
 type TimedHMACValidator struct {
 	authenticatorV2 *authv2.Validator
 	authenticator   *auth.TimedHMACValidator