Add action to label mergeable PRs

[driazati]

Developers often have to ping a committer once their PRs are both passing in CI and are approved. This helps facilitate this process by marking such PRs with a label ready-for-merge so committers can easily filter for outstanding PRs that need attention.

Tested in https://github.com/driazati/tvm/pull/2 via https://github.com/driazati/tvm/actions/workflows/ready_for_merge.yml

[jroesch]

This looks great to me, lmk when this is "ready for merge review" 😆

[areusch]

[DO NOT MERGE] approving to trigger the workflow

[areusch]

clicked wrong button

[driazati]

Reverting back to draft since I'm not sure this is workable since when GitHub reports a status (i.e. when Jenkins is done) I don't see any way to get the original pull request when the PR comes from a forked repo, so the main use case (approval happens, then Jenkins completes hours later) doesn't work. We might just need to push this out of GHA to run at the end of Jenkins once everything is complete for status events this was annoying to implement so I re-worked it to run as a cron (there might be a delay in when the label gets added but it shouldn't be too bad at ~15 min)

[github-actions]

It has been a while since this PR was updated, @areusch please leave a review or address the outstanding comments

[github-actions]

It has been a while since this PR was updated, @areusch please leave a review or address the outstanding comments

[driazati]

closing this to keep it from languishing anymore, an RFC will be coming soon that should address the underlying issue of merging PRs