Add "Manage merge request settings" custom permission

This MR adds Manage merge request settings as a customizable permission, so that it can be added onto any base role.

This makes managing merge request settings on group and/or project level available to all roles, as the Owner role might have too many privileges for all use cases.

Issue: #443235 (closed)

Screenshots or screen recordings

Group member with custom role	Project member with custom role

How to set up and validate locally

As admin:

1. Create a group and apply the Ultimate license to the group
2. If SaaS mode is:
   • off: Visit http://localhost:3000/admin/application_settings/roles_and_permissions and create a custom role with the Manage merge request settings permission enabled, based on Guest access
   • on: Visit http://localhost:3000/groups/${new_group}/-/settings/roles_and_permissions and create a custom role with the Manage merge request settings permission enabled, based on Guest access
3. Invite a user to the group and assign the new custom role to the user

As group member with the custom role:

1. Visit http://localhost:3000/groups/${new_group}/-/edit and verify the page is accessible and you can edit the options in the Merge requests and Merge request approvals sections.

As admin:

1. Create a project in the group
2. Invite another user to the project and assign the new custom role to the user

As project member with the custom role:

1. Visit http://localhost:3000/{new_group}/{new_project}/-/settings/merge_requests and verify the page is accessible and you can manage all settings, create approval rules and settings, and branch targets.