Auth: Introduce pre-logout hooks + add GCOM LogoutHook (#88475)

* Introduce preLogoutHooks in authn service

* Add gcom_logout_hook

* Config the api token from the Grafana config file

* Simplify

* Add tests for logout hook

* Clean up

* Update

* Address PR comment

* Fix

pkg/services/authn/authnimpl/service_test.go

@@ -561,6 +561,7 @@ func setupTests(t *testing.T, opts ...func(svc *Service)) *Service {
 		metrics:                newMetrics(nil),
 		postAuthHooks:          newQueue[authn.PostAuthHookFn](),
 		postLoginHooks:         newQueue[authn.PostLoginHookFn](),
+		preLogoutHooks:         newQueue[authn.PreLogoutHookFn](),
 	}
 
 	for _, o := range opts {