Adopt split repo: _gapic-generator-python_ #14907
Description
Migrate the split-repo https://github.com/googleapis/gapic-generator-python to https://github.com/googleapis/google-cloud-python.
The migration readiness criteria are the following, which we track via GitHub labels on this issue. These criteria apply to the split repo we are migrating from:
No open issues
No open PRs
No handwritten samples