x/build/cmd/gerritbot: support multiple CL authors (Co-authored-by)

[hyangah]

The current gerrit and gerritbot workflow is designed assuming a single author for a CL.
But we observed that often that is not always the case, and see features or changes that
involve multiple authors.

1. a PR is from the results of pair or group programming.
2. the original PR author couldn't continue for some reason so the maintainers or someone else have to finish it before merging.

Currently, gerritbot picks the last committer as the author if a PR involves multiple authors.
(See #38957)

Correctly checking CLA of all involved authors, and attributing contribution to
everyone is important for open source projects.

I don't know if Gerrit supports multi-author workflows.
Github has support for multiple authors using Co-authored-by trailers.
https://github.blog/2018-01-29-commit-together-with-co-authors/
Even when Gerrit wouldn't provide the support, I hope the gerritbot still can ensure
CLA check to be run on all the authors (I think it already does) and include all the
committers (except those merged during rebase?) with the Co-authored-by trailers
before squashing.

cc @dmitshur @golang/osp-team