Async Core API: Users

Implement async versions of user CRUD and properties operations (11 methods total).

Methods include: get_users, get_user, create_user, update_user, delete_user, get_user_property, set_user_property, delete_user_property, get_user_collab_groups, set_user_collab_groups, delete_user_collab_groups.

See planned methods list in /Users/borja.toron/.claude/plans/magical-bouncing-curry.md