Auth: Add IsClientEnabled and IsEnabled for the authn.Service and authn.Client interfaces (#86034)

* Add `Service. IsClientEnabled` and `Client.IsEnabled` functions

* Implement `IsEnabled` function for authn clients

* Implement `IsClientEnabled` function for authn services

pkg/services/authn/clients/api_key.go

@@ -70,6 +70,10 @@ func (s *APIKey) Authenticate(ctx context.Context, r *authn.Request) (*authn.Ide
 	return newServiceAccountIdentity(key), nil
 }
 
+func (s *APIKey) IsEnabled() bool {
+	return true
+}
+
 func (s *APIKey) getAPIKey(ctx context.Context, token string) (*apikey.APIKey, error) {
 	fn := s.getFromToken
 	if !strings.HasPrefix(token, satokengen.GrafanaPrefix) {