IRCv3: Add an account option to specify the SASL mechanisms
This allows the user to only try the SASL mechanism they want to use instead of
potentially trying a few that won't work based on what the server advertised.
This also allows us to use SASL mechanisms that the server supports but doesn't
advertise for some reason.
Testing Done:
Set the field to `SCRAM-SHA-256` and connected to my local ergo. I verified that ONLY `SCRAM-SHA-256` was attempted (and failed see [PIDGIN-17744](https://issues.imfreedom.org/issue/PIDGIN-17744).
I deleted the value from the entry as well as from accounts.xml and verified that the server advertised values were what were used.