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

builder/cmd/main.go

@@ -38,5 +38,8 @@ func main() {
 	builder := builder.MakeBuilder(dir)
 	mux := http.NewServeMux()
 	mux.HandleFunc("/", builder.Handler)
+	mux.HandleFunc("/healthz", func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	})
 	http.ListenAndServe(":8001", mux)
 }