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