Support Reporting CLA Status on Merge Queue Events

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We want to enable merge queues. But for that to work, all required checks must pass on the items in the merge queue independently of them passing on the PR. This means for the CLA check, we need to listen for items being added to the merge queue and pass back a CLA check status on them.

Notes for the implementer

This might be a bit complicated because the merge queue does not have a link back to the PR in the webhook payload: https://docs.github.com/en/webhooks/webhook-events-and-payloads#merge_group

We may need to check that the CLA check passes on the author of each commit in the merge_group instead.