input: Write a python function to get a list of users' names from the UsersDB class.
  ```python
  assert get_users_names(users_db) == ["John Doe", "Jane Doe"]
  users_db.add_user(User("Alice Johnson", 30, "1 Infinite Loop, Cupertino, CA"))
  assert get_users_names(users_db) == ["John Doe", "Jane Doe", "Alice Johnson"]
output: def get_users_names(users_db):
      return [user.name for user in users_db.get_all_users()]