Support Reporting CLA Status on Merge Queue Events

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.

Start by reviewing the webhook handling for GitHub merge_group events and the merge_group payload documentation linked in the issue. Determine how CLA status can be reported for queued items without a pull-request link, including the listed commit authors; done means required CLA checks are returned for merge queue items.