Customer custom MR widgets/Checks
## Overview
Holding epic for issues related to customer created MR widgets / Checks API work. Likely overlap with ~"group::code review"'s work on [Better Defined mergeability](https://gitlab.com/groups/gitlab-org/-/epics/5598).
## Problem
Customers use tools (linters, scanners, CI even) outside of GitLab that influence if an MR should merge or not. They cannot reflect status of those things in an MR today in a way that's obvious an MR should not merge.
## Target outcome
As a developer, I can see if an MR can merge or not, link to tooling that gives me the "why not" and resolve it, so I can work efficiently and effectively in GitLab/other tools.
epic