Add backport GHA workflow

[tomleb]

Context

Backporting PRs to previous branch is pretty tedious. You have to:

• git fetch
• Checkout the target branch
• Create a new backport branch
• Find the commit hash of the merged PR
• cherry-pick the commit
• Fix or adjust the commit if necessary
• git push
• Go to the repository in your browser
• Create a pull request
  • Usually the name of the branch will look something like [<branch>] <title of PR to backport>
  • Usually the description will be the same because the changes are the same. A backlink to the original PR is useful for reviewers so personally I add it whenever I think about it.
  • Usually I assign the same reviewers that were in the original PR
  • ^This is tedious

Summary

This PR is just an idea of a workflow that does all of this for us. The idea is you can just click on the Backport PR GHA workflow. Here's a screenshot of the workflow where the branch selected would be the target, the user must specify which PR number to backport and whether or not to cherry-pick the commit (the idea here is that we would still gain from everything even if we create an empty commit that can be edited).

Here's an example result from just running the script: #508.

Here's an example result from running in GHA: tomleb#11. Pretty much the same thing.

Note: I'd like some input from the team to know whether that's something they'd be interested in. I can totally run this script locally but if the whole team can benefit from it then why not.

Note: This could also be in the form of a github bot that reacts to comments, though that involves more work. (eg: Image just having to write /backport release/v0.4 in a merged PR.

Note: The script only supports squash commits. Now that squashed commits are enforced, this shouldn't be an issue in the future.

Possible improvements

• Ability to assign it to the same people that reviewed the original PR automatically. Currently it's not done because the PR could fail to cherry-pick, though I could see it being useful if the cherry-pick succeeded.