Support message retraction for XMPP

[lukasjuhrich]

Is your feature request related to a problem? Please describe.
The xmpp backend does not support forwarding message retraction (aka “deletion”) from other backends, such as a deleted mattermost message.

Describe the solution you'd like
Support of message retraction by means of XEP-0424: Message Retraction when it is supported.
Even if in principle, you cannot force an XMPP client to adhere to the retraction, it would still be nice if a retraction message would be sent out.

Describe alternatives you've considered
–

Additional context
–

[lukasjuhrich]

My apologies, I've just noted XEP-0424 is only proposed and not stable.