sasl: Support certfp

[xiu]

We currently support SASL PLAIN for password authentication though users have expressed interest around cert based authentication.

Lit of tasks as I think of it at the moment:

• Add a subcommand to nick to store the certfp (/msg C nick set certfp?). Store it in db.
  • Check that it is possible to add a subcommand from a different submodule as m_nick
• Provide migrations to add the new column
• Add logic to support SASL mechanism EXTERNAL
• Add tests