Reads GRR users with optional pagination, sorted by username.

def ReadGRRUsers(self, offset=0, count=None):
    """Reads GRR users with optional pagination, sorted by username."""
    if count is None:
      count = len(self.users)

    users = sorted(self.users.values(), key=lambda user: user.username)
    return [user.Copy() for user in users[offset:offset + count]]