Custom role optimizations/caching

[laurenleach]

• Move custom role grants to group grants to avoid making duplicate requests for lists groups/list group assignments
• Remove administrator logic as it does not apply to custom roles
• Find users by list users with role assignments instead of listing all users
• Cache user role assignments so we don't have duplicate requests for listing user role assignments per role
• Get ResourceSet custom role grants from role resource set bindings api so we don't have to list all users and all assigned roles per user for each resource set