ci: create way to trigger "merge when ready" without admin intervention

[ryannikolaidis]

Is your feature request related to a problem? Please describe.
With merge queues enabled, now an admin must be the one to trigger the "Merge when ready" button on a PR, in order to add it to the merge queue.

Describe the solution you'd like
Ideally if a PR has been approved that should be sufficient to allow merging or rather "adding to the queue to merge". It doesn't seem this is exposed as an option in GH repo settings.

Describe alternatives you've considered
Potentially this could be done with a bot or possibly a workflow trigger?

Additional context
It looks like we can add to the merge queue via the github cli. Basically if merge queue is required/enabled gh pr merge will add it to the queue rather than merging. This could be leveraged for either of the proposed options.