"Enable" and support using of wildcard certificates for Mail Protection/SMTP Proxy.

Explanation:
We had a case where we used our existing official wildcard certificate also for our SMTP connection. There was no error importing and configuring it on the webinterface and most of the connections worked but some are not. Funny part of the story: also the Sophos Support salesforce mails didn't go through ;-) So as we use this certificate for nearly all our external connections it would be fine if we wouldn't need another one for the Sophos UTM. Technically I don't see the problem and half of the work seems to be done because it is working basically. There is only a special thing which is not working and this one is not revised because of this KB: https://support.sophos.com/support/s/article/KB-000034685?language=en_US...