IDforwarding: forward signed id to plugins (#75651)

* Plugins: Add client middlware that forwards the signed grafana id token if present

* DsProxy: Set grafana id header if id token exists

* Add util function to apply id token to header

* Only add id forwarding middleware if feature toggle is enabled

* Add feature toggles to ds proxy and check if id forwarding is enabled

* Clean up test setup

* Change to use backend.ForwardHTTPHeaders interface

* PluginProxy: Forward signed identity when feature toggle is enabled

* PluginProxy: forrward signed id header

pkg/services/pluginsintegration/pluginsintegration.go

@@ -167,6 +167,10 @@ func CreateMiddlewares(cfg *setting.Cfg, oAuthTokenService oauthtoken.OAuthToken
 		middlewares = append(middlewares, clientmiddleware.NewCachingMiddlewareWithFeatureManager(cachingService, features))
 	}
 
+	if features.IsEnabled(featuremgmt.FlagIdForwarding) {
+		middlewares = append(middlewares, clientmiddleware.NewForwardIDMiddleware())
+	}
+
 	if cfg.SendUserHeader {
 		middlewares = append(middlewares, clientmiddleware.NewUserHeaderMiddleware())
 	}