Implement XEP-0480: SASL Upgrade Tasks

[tmolitor-stud-tu]

This is the last missing piece for modern SASL2 authentication: XEP-0480: SASL Upgrade Tasks is needed to make sure clients can update the old SHA-1 password hashes to more secure alternatives like SHA-256.

This isn't as urgent as the other SASL2 related stuff you just implemented, but needed to make sure we can transition from SHA-1 to something more secure before SCRAM-SHA-1 becomes insecure.
This transition will take quite some time, so it is good to start early with this.

I promise this is the last SASL-related implementation request I'm doing ;)

BTW: This XEP was originally developed inside the main SASL2 XEP (XEP-0388) but later factored out to not create another of these large XEPs like MUC or PubSub.

[tmolitor-stud-tu]

Side note: I've wrote a blog post about modern SASL authentication that more or less led to all of these SASL2 related XEPs over here: https://monal-im.org/post/00004-sasl/

[Neustradamus]

Important too :)