github: Add actions to automate part of the release workflow

This adds support for automatically cherry-picking and testing fixes for the
release branch using 'commands' in issue comments.  The two supported commands are:

/cherry-pick <commit1> <commit2> ...

Which will backport and test commits from main.  And also

/branch owner/repo/branch

Which will test commits from the given branch.

Reviewed By: alexbatashev, kwk

Differential Revision: https://reviews.llvm.org/D117386