Add ability to associate customizable roles with static roles via API

2 approaches could work:

Option 1

First approach would be similar to: #370090 (closed); we would create entirely new API endpoints that find custom role and associate it with an existing static role. Something like:

PATCH/PUT /groups/:id/member_roles where you would pass a user and namespace as params and it would find the member record for that user + namespace and associate it with the member_role

Option 2

Second approach would be update the existing API endpoints PUT /groups/:id/members/:user_id and PUT /projects/:id/members/:user_id so that they can be used to update a member record and add a member_role_id.

Note: Please add the ability to pass the Users to be assigned the role as a list. We have an early adopter who would like to assign this to 800 users. Doing it 1 by 1 is not scalable.