server: Impossible to disable PLAIN auth

[kayrus]

PLAIN auth is enabled by default and it is not possible to disable it:

go-smtp/server.go

Lines 86 to 87 in 30169ac

	auths: map[string]SaslServerFactory{
	sasl.Plain: func(conn *Conn) sasl.Server {

it also doesn't look correct that Session interface now requires the AuthPlain method (#146). Why doesn't it require other auth methods? Why don't rely on sasl auth methods defined explicitly?

[emersion]

go-imap v2's design: https://pkg.go.dev/github.com/emersion/go-imap/v2/imapserver#SessionSASL

[emersion]

#254 fixes this.