input: Write a python function to start a Flask app that listens to a specific port and handles a POST request.
  assert start_flask_app(5000) == 'App started on port 5000'
  assert start_flask_app(8080) == 'App started on port 8080'
  assert start_flask_app(3000) == 'App started on port 3000'
output: from flask import Flask, request, jsonify
  
  def start_flask_app(port):
      app = Flask(__name__)
  
      @app.route('/post', methods=['POST'])
      def handle_post():
          data = request.get_json()
          return jsonify(data), 200
  
      def run_app():
          app.run(port=port)
  
      import threading
      threading.Thread(target=run_app).start()
      return f'App started on port {port}'