github: Automatically assign reviewers for backport requests

github: Automatically assign reviewers for backport requests

When there is a backport request, the GitHub Action that handles the
backport will now automatically assign the issue to the user(s) who
approved the commit in Phabricator and create an issue comment asking
them to review the request.

Reviewed By: thieta, kwk

Differential Revision: https://reviews.llvm.org/D126423