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', email='john@example.com') == {"message": "User created"}
  assert get_users() == [{"id": 1, "name": "John", "email": "john@example.com"}]
  assert get_user(id=1) == {"id": 1, "name": "John", "email": "john@example.com"}
output: users = []
  user_id_counter = 1
  
  def create_user(name, email):
      global user_id_counter
      user = {"id": user_id_counter, "name": name, "email": email}
      users.append(user)
      user_id_counter += 1
      return {"message": "User created"}
  
  def get_users():
      return users
  
  def get_user(id):
      for user in users:
          if user["id"] == id:
              return user
      return None