Implement SASL 2

[lnjX]

• https://xmpp.org/extensions/xep-0388.html
• #606

Implementation todos:

• [x] parsing of Sasl2 packets
• [x] minimal Sasl2Manager
• [x] integration into OutgoingClient
• [ ] send UserAgent and add options to QXmppConfiguration
• [ ] interface for registering extensions for SASL 2 tasks?
• [ ] Sasl2Manager unit test with
  • [ ] SCRAM auth
  • [ ] failure
  • [ ] <continue/> with required tasks