Add a corpus of generated protobuf messages generated at specific versions
of protoc-gen-go to ensure that we continue to support for generated messages
that have may never be updated.
Change-Id: I04a1b74306f471d7c99f5daf52399a5bd9adcbbc
Reviewed-on: https://go-review.googlesource.com/c/148831
Reviewed-by: Herbie Ong <herbie@google.com>