Allow filtering by `approved_by_username` in Group Merge Requests API

In https://docs.gitlab.com/ee/api/merge_requests.html#list-group-merge-requests, currently there is no way to filter MRs by approved_by_username.

There is support for filtering by reviewer_username, assignee_username and author_username, but ability to filter by approved_by_username is missing.

Requirement:

Allow filtering by approved_by_username, so that the following API call is valid:

https://gitlab.com/api/v4/groups/<group_id>/merge_requests?approved_by_username=<username>

Usecase:

This specific requirement came up in https://gitlab.com/gitlab-org/gitlab-roulette/-/merge_requests/59 so as to filter MRs that someone has already approved.