Add initial implementation of protoc-gen-go plugin.

  * (A) protoyangplugin
    - This PR adds an initial implementation of a protoc-gen-go plugin
      which is able to extract the yext schema path annotation from an
      input proto file and output a map of the path to types.

protoyangplugin/pkg/testproto/test.proto

@@ -0,0 +1,35 @@
+// Copyright 2018 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+syntax = "proto3";
+
+package test;
+import "github.com/openconfig/ygot/proto/yext/yext.proto";
+
+
+message AMessage {
+  BMessage set = 1 [(yext.schemapath) = "/b"];
+  BMessage unset = 2;
+  CMessage nested = 3 [(yext.schemapath) = "/b/c"];
+  message CMessage {
+    DMessage nested = 4 [(yext.schemapath) = "/b/c/d"];
+    message DMessage {
+      string field = 1 [(yext.schemapath) = "/b/c/d/field"];
+    }
+  }
+}
+
+message BMessage {
+  string field = 1;
+}