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