Add a way to get the content to the `Message` interface

[Ran-Mewo]

Is your feature request related to a problem? Please describe.
It's impossible to store all types of messages and get their content

Describe the solution you'd like
A some sort of getStringContent() method to the Message interface which will return the content of any form of message, for the UserMessage it'll return the content if the content is a string, otherwise if it's a Content it'll return the text content or the image url