Improve list groups roles performance

Uses GET v3/role_assignments?scope.project.id=<project_id>
to fetch groups roles in a given project, instead of doing
an individual call for every known group.

Closes-Bug: 1369137
Change-Id: Ie64dfddc0d8cbb4f22f3ee3b19c248ca6973e96c
Merge "Improve list groups roles performance"