Allow groups in reviewers field

Context

Currently our Reviewer field does not support assigning a group handle - hence requesting a review from group. We have to assign individuals one by one.

One of our customers, to whom I'm in contact with, requested this feature as they usually assign a group, and the first person who's available picks up the MR to review. Currently they assign multiple team members at once.

The whole developer team size is more than 30 people but they usually work in smaller chunks of groups.

On other side, I came across with this issue: #197169 (closed) and it seems like this is a heavily requested feature.

Recently I did the same to ask for a UX review: !87234 (comment 942708034)

Feature Request

It'd nice to be able to assign a group in the Reviewer field and all members will be pinged. The initial flow could be that the person who picks the MR to review unassigns the group and then assigns themselves so multiple people do not review at the same time (or in some case this is also desirable).