Rendering: Store render key in remote cache (#22031)

By storing render key in remote cache it will enable
image renderer to use public facing url or load
balancer url to render images and thereby remove
the requirement of image renderer having to use the
url of the originating Grafana instance when running
HA setup (multiple Grafana instances).

Fixes #17704
Ref grafana/grafana-image-renderer#91

pkg/api/common_test.go

@@ -143,7 +143,7 @@ func setupScenarioContext(url string) *scenarioContext {
 		Delims:    macaron.Delims{Left: "[[", Right: "]]"},
 	}))
 
-	sc.m.Use(middleware.GetContextHandler(nil, nil))
+	sc.m.Use(middleware.GetContextHandler(nil, nil, nil))
 
 	return sc
 }