Create Projects::BranchRules::MergeRequestApprovalSetting model

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/183679



Merged-by: Javiera Tapia <jtapia@gitlab.com>
Approved-by: Fabien Catteau <fcatteau@gitlab.com>
Reviewed-by: Fabien Catteau <fcatteau@gitlab.com>
Reviewed-by: Jerry Seto <jseto@gitlab.com>