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.

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.

I deleted the value from the entry as well as from accounts.xml and verified that the server advertised values were what were used.