Implement modify data message
Created by: claucece
Description: Modify function should receive the data message and modify the text in it.
For that the user must to give the MAC key for that message (in order to generate a new mac for the modified message).
Reference: https://bugs.otr.im/lib/libotr/blob/master/README (TOOLKIT section) https://bugs.otr.im/lib/libotr/tree/master/toolkit https://github.com/otrv4/otrv4/blob/master/otrv4.md#forging-transcripts https://en.wikipedia.org/wiki/Malleability_(cryptography)