feat: add multiple authentication methods, token and oidc.

token is the current token comparison, and oidc generates oidc token using client-credentials flow. in addition - add ping verification using the same method

vendor/google.golang.org/appengine/internal/main.go

@@ -0,0 +1,16 @@
+// Copyright 2011 Google Inc. All rights reserved.
+// Use of this source code is governed by the Apache 2.0
+// license that can be found in the LICENSE file.
+
+// +build appengine
+
+package internal
+
+import (
+	"appengine_internal"
+)
+
+func Main() {
+	MainPath = ""
+	appengine_internal.Main()
+}