Support `merge_group` events for `type=ref` tags

[psypuff]

Description

Github Merge Queue is a common part of a CI/CD flow nowadays. In our case the test suite triggered by the Merge Queue relies on the existence of a Docker image. For us it only makes sense to tag it with the PR number that went into the Merge Queue so we could also benefit from layer cache later on.

It'd be really nice to have a type=ref,event=mq tag option, similar to the type=ref,event=pr one.
Currently, our workaround is type=raw,value=pr-${{ <PR Number> }},enable=${{ github.event_name == 'merge_group' }} where the PR number is extracted from github.event.merge_group.head_ref.