Add appengine/gcloud/mustache dependencies.

pkg/appengine/lib/src/protobuf_api/internal/remote_api.proto

+syntax = "proto2";
+
+package remote_api;
+
+message Request {
+  required string service_name = 2;
+  required string method = 3;
+  required bytes request = 4;
+  optional string request_id = 5;
+}
+
+message ApplicationError {
+  required int32 code = 1;
+  required string detail = 2;
+}
+
+message RpcError {
+  enum ErrorCode {
+    UNKNOWN = 0;
+    CALL_NOT_FOUND = 1;
+    PARSE_ERROR = 2;
+    SECURITY_VIOLATION = 3;
+    OVER_QUOTA = 4;
+    REQUEST_TOO_LARGE = 5;
+    CAPABILITY_DISABLED = 6;
+    FEATURE_DISABLED = 7;
+    BAD_REQUEST = 8;
+    RESPONSE_TOO_LARGE = 9;
+    CANCELLED = 10;
+    REPLAY_ERROR = 11;
+    DEADLINE_EXCEEDED = 12;
+  }
+  required int32 code = 1;
+  optional string detail = 2;
+}
+
+message Response {
+  optional bytes response = 1;
+  optional bytes exception = 2;
+  optional ApplicationError application_error = 3;
+  optional bytes java_exception = 4;
+  optional RpcError rpc_error = 5;
+}