input: Write a python function to send and retrieve messages using Flask API.
  assert send_message({'message': 'Hello'}) == {'message': 'Message successfully stored'}
  assert get_messages() == [{'message': 'Hello'}]
  assert send_message({'message': 'How are you?'}) == {'message': 'Message successfully stored'}
  assert get_messages() == [{'message': 'Hello'}, {'message': 'How are you?'}]
output: from flask import Flask, request, jsonify
  
  app = Flask(__name__)
  messages = []
  
  @app.route('/send_message', methods=['POST'])
  def send_message():
      data = request.get_json()
      messages.append(data)
      return jsonify({'message': 'Message successfully stored'})
  
  @app.route('/get_messages', methods=['GET'])
  def get_messages():
      return jsonify(messages)
  
  if __name__ == '__main__':
      app.run(debug=True)