feat: add support for merge queues (merge_group events)

[janeklb]

Motivation:
nx-set-shas lacks support for github's merge queues

Modification:

• when eventName === 'merge_group' and merge-queue-base-from-pr input set to true, set the BASE_SHA to the merge-base of the the default branch + PR's branch

Notes / disclaimer:

• extracting the PR's ref is a little brittle at the moment, since the merge_group webhook payload does not include it explicitly
  • raised a request to github to include more metadata in the payload: https://github.com/orgs/community/discussions/62219
• using merge-queue-base-from-pr on merge queues with the limit set greater than 1 is not recommended (will probably not work as expected).
• did not set up a test for this since it would require merge queues enabled on this repo (if the current testing pattern were to be followed)

[janeklb]

likely addresses #90 (though I haven't gone through it closely)

[andersonba]

up!

[meeroslav]

Thank you for this PR!

[janeklb]

Thanks @meeroslav!

I see that you've removed the option to opt-in to the special merge_group behaviour -- do you not consider there to be any risk running this against merge queue builds/branches that include multiple PRs?

[meeroslav]

So far, we have not had proper support for merge_group, but now thanks to you we do. I would rather have it on so we can polish any issues early on, rather than have it opt in a drag potential issues for months.

[janeklb]

The opt in flag was for the special "branch name parsing to get the merge base" logic.

Imo generic merge group behaviour should be enabled and available (by default / always - in the same was as pull requests), but it's this extra bit I was asking about