Add support in protoc-gen-cppstream for optional fields in proto3

This support is optional. experimental_allow_proto3_optional must be
included in the --cppstream_opt field to enable this.

Test: m -j frameworks/base/tools/streaming_proto
Fixes: 291329285
Change-Id: I7b985b84edf0cda46d06a9ebc39df574695ef418
diff --git a/tools/streaming_proto/OWNERS b/tools/streaming_proto/OWNERS
new file mode 100644
index 0000000..5f6e59f
--- /dev/null
+++ b/tools/streaming_proto/OWNERS
@@ -0,0 +1 @@
+mwachens@google.com