input: Write a function called `start_flask_app` that initializes a Flask app to listen for POST requests on the '/webhook' route. When a request is received, it should extract the necessary parameters from the request and call a `makeWebhookResult` function to generate a response. The response should be a JSON object containing a 'speech' field with the text to be spoken by the voice assistant. The app should run on port 5000 and be started in debug mode.
  assert start_flask_app().app.url_map._rules[1].rule == '/webhook'
  assert start_flask_app().app.url_map._rules[1].methods == {'POST'}
  assert start_flask_app().name == '__main__'
  assert start_flask_app().debug
output: def start_flask_app():
      from flask import Flask, request, jsonify
      app = Flask(__name__)
  
      @app.route('/webhook', methods=['POST'])
      def webhook():
          params = request.get_json()
          response = makeWebhookResult(params)
          return jsonify(response)
  
      if __name__ == '__main__':
          app.run(port=5000, debug=True)
      return app