Ability to check `git config user.name` with GitLab full name in push rules
### Problem to solve
Under some strict compliance requirements of companies, the commit author info in git log should be consistent with those on GitLab.
However, currently it is allowed that user can set `git config user.name` different from GitLab's profile name, because we only check the user email in our push rules. This could be troublesome to company compliance management.

### Proposal
<!-- Use this section to explain the feature and how it will work. It can be helpful to add technical details, design proposals, and links to related epics or issues. -->
In **project settings > repository > push rules > select push rules**, add a new option `Check the commit author git.username is a GitLab user`
Since this is a company-compliance-related feature, we hope it is priced for ~"GitLab Premium"
issue