Currently, this POST endpoint allows creating an SVG grid of cover art from release MBIDs: https://listenbrainz.readthedocs.io/en/latest/users/api/art.html#post--1-art-grid-

It would be useful to be able to pass an array of release group MBIDs for convenience in a separate `release_group_mbids` array.

This would allow users to get cover art without requiring to know which of the releases from that release group actually have cover art attached to it.

I would suggest throwing an error if the user supplies both arrays, requiring either one or the other, for simplicity.
But maybe that's over conservative and we can process both concurrently.