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_common.go

@@ -0,0 +1,7 @@
+package internal
+
+// MainPath stores the file path of the main package. On App Engine Standard
+// using Go version 1.9 and below, this will be unset. On App Engine Flex and
+// App Engine Standard second-gen (Go 1.11 and above), this will be the
+// filepath to package main.
+var MainPath string