input: Write a python function to create a Person instance from a JSON object.
  assert str(create_person_instance('{"first_name": "John", "last_name": "Smith"}')) == "John Smith"
  assert str(create_person_instance('{"first_name": "Jane", "last_name": "Doe"}')) == "Jane Doe"
  assert str(create_person_instance('{"first_name": "Alice", "last_name": "Johnson"}')) == "Alice Johnson"
output: import json
  
  class Person:
      def __init__(self, first_name, last_name):
          self.first_name = first_name
          self.last_name = last_name
  
      def __str__(self):
          return f"{self.first_name} {self.last_name}"
  
  def create_person_instance(json_str):
      data = json.loads(json_str)
      return Person(data['first_name'], data['last_name'])