Initial distributor implementation

common/api/dm/distributor/distributor.proto

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+syntax = "proto3";
+
+package distributor;
+
+message Alias {
+  string other_config = 1;
+}
+
+message Distributor {
+  reserved 2; // future: generic pRPC based distributor
+  reserved 3; // future: generic gRPC based distributor
+
+  reserved 4; // future swarming_v1 distributor
+  reserved 2048; // future jobsim distributor
+
+  // TODO(iannucci): Maybe something like Any or extensions would be a better
+  // fit here? The ultimate goal is that users will be able to use the proto
+  // text format for luci-config. I suspect that Any or extensions would lose
+  // the ability to validate such text-formatted protobufs, but maybe that's
+  // not the case.
+  oneof distributor_type {
+    Alias alias = 1;
+  }
+}
+
+message Config {
+  map<string, Distributor> distributor_configs = 1;
+}