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
• [x] send UserAgent and add options to QXmppConfiguration
• [x] Sasl2Manager unit test with
  • [x] failure
  • [x] <continue/> with required tasks
  • [x] disallow PLAIN auth

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