Support TOTP 2FA for Stalwart

[mitchplze]

The TOTP 2FA plugin in Snappymail does not support Stalwart mail server MFA.

It would be great if compatibility with their OAuth / OAUTHBEARER functionality could be added.

As a workaround, Stalwart allows you to specify the TOTP code at the end of the password field (separated with $), but this is probably not the ideal way to implement this. When I currently use this method (plugin disabled), my session times out pretty quick.