Add envoy and its dependent repos as submodules in third_party

The basic criteria is to have protoc generate code for envoy api's and
this change achieves that.

e.g.
protoc -I=$PWD -I=$PWD/third_party/data-plane-api -I=$PWD/third_party/googleapis -I=$PWD/third_party/protobuf -I=$PWD/third_party/protoc-gen-validate third_party/data-plane-api/envoy/api/v2/eds.proto

.gitmodules

@@ -42,3 +42,12 @@
 	path = third_party/libcxx
 	url = https://github.com/llvm-mirror/libcxx.git
 	branch = release_60
+[submodule "third_party/data-plane-api"]
+	path = third_party/data-plane-api
+	url = https://github.com/envoyproxy/data-plane-api.git
+[submodule "third_party/googleapis"]
+	path = third_party/googleapis
+	url = https://github.com/googleapis/googleapis.git
+[submodule "third_party/protoc-gen-validate"]
+	path = third_party/protoc-gen-validate
+	url = https://github.com/lyft/protoc-gen-validate.git