Block build requests until environment builder is ready (#437)

* Add readiness probe
* Remove builder manager http api interface since we don’t use/need it
* Check environment builder status and block build requests until builder is ready
* Replace deprecated api extension interface
* Add healthy check to python env

environments/fetcher/cmd/main.go

@@ -23,5 +23,8 @@ func main() {
 	mux := http.NewServeMux()
 	mux.HandleFunc("/", fetcher.FetchHandler)
 	mux.HandleFunc("/upload", fetcher.UploadHandler)
+	mux.HandleFunc("/healthz", func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	})
 	http.ListenAndServe(":8000", mux)
 }