Add impersonate responder function (Non-Interactive)

[annacruz]

Description:
The function to impersonate the responder should receive a long-term secret key, and with this should change the responder to use this key (by executing a Rsig)

References:
https://github.com/otrv4/otrv4/blob/master/otrv4.md#forging-transcripts
https://github.com/otrv4/otrv4/blob/master/otrv4.md#ring-signature-authentication

[claucece]

This is not waiting from a decision from anyone. It can be implemented right now :)