Add a separate `sendingFqdn` option distinct from `fqdn`

Though it won't reduce your score, one of mail-tester's tests is to check that the FQDN passed in the EHLO to the receiving server matches the rDNS of the server's IP. This configuration is achievable with the module as stands (using services.postfix.hostname = lib.mkForce sendingFqdn; to override the configuration from mailserver) but I think it's good to make it more officially/easily available and document the possibility.

I'm not sure if this is the right way to go about it — whether fqdn is supposed to refer to the user-facing FQDN or the internal sending FQDN. I've assumed the former here just because there are more uses of it as the user-facing FQDN, but I'd appreciate guidance :)