Add PATCH /groups/:id/members/:user_id/member_role

What does this MR do and why?

• This EE-only API endpoint will be used to associate group members with custom roles.
• We will want this for project members too but this endpoint is a good first iteration.
• Custom roles functionality is still behind the customizable_roles feature flag
• #383014 (closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. Enable the customizable_roles feature flag for the top-level group for which you are the owner and plan to create custom roles.
2. Create the custom GUEST role for the group via the API:

    curl --request POST --header "Content-Type: application/json" --header "Authorization: Bearer $YOUR_ACCESS_TOKEN" --data '{"base_access_level":10, "download_code":1}' "http://localhost:3000/api/v4/groups/$GROUP_PATH/member_roles"

3. Invite a test user account to the root group as a GUEST
4. Associate membership with a custom role using the API endpoint added in this MR

    curl --request PATCH --header "Content-Type: application/json" --header "Authorization: Bearer $YOUR_ACCESS_TOKEN" --data '{"member_role_id":$MEMBER_ROLE_ID}' "http://localhost:3000/api/v4/groups/$GROUP_ID/members/$GUEST_USER_ID/member_role"

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.