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/urlfetch/urlfetch_service.proto

@@ -0,0 +1,64 @@
+syntax = "proto2";
+option go_package = "urlfetch";
+
+package appengine;
+
+message URLFetchServiceError {
+  enum ErrorCode {
+    OK = 0;
+    INVALID_URL = 1;
+    FETCH_ERROR = 2;
+    UNSPECIFIED_ERROR = 3;
+    RESPONSE_TOO_LARGE = 4;
+    DEADLINE_EXCEEDED = 5;
+    SSL_CERTIFICATE_ERROR = 6;
+    DNS_ERROR = 7;
+    CLOSED = 8;
+    INTERNAL_TRANSIENT_ERROR = 9;
+    TOO_MANY_REDIRECTS = 10;
+    MALFORMED_REPLY = 11;
+    CONNECTION_ERROR = 12;
+  }
+}
+
+message URLFetchRequest {
+  enum RequestMethod {
+    GET = 1;
+    POST = 2;
+    HEAD = 3;
+    PUT = 4;
+    DELETE = 5;
+    PATCH = 6;
+  }
+  required RequestMethod Method = 1;
+  required string Url = 2;
+  repeated group Header = 3 {
+    required string Key = 4;
+    required string Value = 5;
+  }
+  optional bytes Payload = 6 [ctype=CORD];
+
+  optional bool FollowRedirects = 7 [default=true];
+
+  optional double Deadline = 8;
+
+  optional bool MustValidateServerCertificate = 9 [default=true];
+}
+
+message URLFetchResponse {
+  optional bytes Content = 1;
+  required int32 StatusCode = 2;
+  repeated group Header = 3 {
+    required string Key = 4;
+    required string Value = 5;
+  }
+  optional bool ContentWasTruncated = 6 [default=false];
+  optional int64 ExternalBytesSent = 7;
+  optional int64 ExternalBytesReceived = 8;
+
+  optional string FinalUrl = 9;
+
+  optional int64 ApiCpuMilliseconds = 10 [default=0];
+  optional int64 ApiBytesSent = 11 [default=0];
+  optional int64 ApiBytesReceived = 12 [default=0];
+}