Add support to upload files using XEP-0363

[srgcdev]

It would be nice to add support for upload files using this xep:
https://xmpp.org/extensions/xep-0363.html

Basically consists in upload the file to the xmpp server and share de url link.
Some xmpp clients supports this xep (Conversations, gajim, dino, movim...)

Thanks in advance.

[mdosch]

I implemented http-upload myself, maybe you can re-use some of that stuff: https://salsa.debian.org/mdosch/go-sendxmpp/-/blob/master/httpupload.go