input: Write a function to implement a REST API in Python for creating and reading users. The API should have endpoints for creating a user, reading all users, and reading a single user.
  assert create_user({'name': 'John Doe', 'email': 'john@example.com'})=={'message': 'User created'}
  assert get_users()==[{'id': 1, 'name': 'John Doe', 'email': 'john@example.com'}]
  assert get_user(1)=={'id': 1, 'name': 'John Doe', 'email': 'john@example.com'}
output: users = []
  user_id_counter = 1
  
  def create_user(user_data):
      global user_id_counter
      user = user_data.copy()
      user['id'] = user_id_counter
      users.append(user)
      user_id_counter += 1
      return {'message': 'User created'}
  
  def get_users():
      return users
  
  def get_user(user_id):
      for user in users:
          if user['id'] == user_id:
              return user
      return None