google-cloud-go: design doc for migrating go-genproto

The https://github.com/googleapis/go-genproto repo *is* generated, but isn't currently generated by OwlBot. Regeneration is run locally, I believe. (The repo doesn't have any releases, so it's not blocking the turndown of release-please.) It would be good to make it generated by Librarian - but this will take a bit of work in the Go container. We have a prototype, but it would be good to go through a more thorough design.

(Once done, this will replace https://github.com/googleapis/google-cloud-go/tree/main/internal/gapicgen - which, contrary to the name, doesn't actually generate GAPIC libraries.)