Support getting one user's effective roles in V3

In V3, we cannot get one user's "effective" roles. Now, we can only get it from /v3/role_assignment, which may be too much for the user. Of course, we cannot change the meaning of the current API, like /domains/{domain}/users/{user}/roles, which returns only explicit roles of the user, and will not return the effective roles (like from the group). So, I propose blueprints of some new APIs, which are /projects/{project_id}/users/{user}/roles?effective and /domains/{domain}/users/{user}/roles?effective. If so, it would be very convenient to get one user's effective roles.