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/authn.go

@@ -72,6 +72,7 @@ type FetchPermissionsParams struct {
 
 type PostAuthHookFn func(ctx context.Context, identity *Identity, r *Request) error
 type PostLoginHookFn func(ctx context.Context, identity *Identity, r *Request, err error)
+type PreLogoutHookFn func(ctx context.Context, requester identity.Requester, sessionToken *usertoken.UserToken) error
 
 type Service interface {
 	// Authenticate authenticates a request
@@ -88,7 +89,8 @@ type Service interface {
 	RedirectURL(ctx context.Context, client string, r *Request) (*Redirect, error)
 	// Logout revokes session token and does additional clean up if client used to authenticate supports it
 	Logout(ctx context.Context, user identity.Requester, sessionToken *usertoken.UserToken) (*Redirect, error)
-
+	// RegisterPreLogoutHook registers a hook that is called before a logout request.
+	RegisterPreLogoutHook(hook PreLogoutHookFn, priority uint)
 	// ResolveIdentity resolves an identity from org and namespace id.
 	ResolveIdentity(ctx context.Context, orgID int64, namespaceID NamespaceID) (*Identity, error)