acme: Add new option acmeCertificateName

Allow the user to specify the name of the ACME configuration that the
mailserver should use. This allows users that request certificates that
aren't the FQDN of the mailserver, for example a wildcard certificate.

default.nix

@@ -675,6 +675,19 @@ in
       '';
     };
 
+    acmeCertificateName = mkOption {
+      type = types.str;
+      default = cfg.fqdn;
+      example = "example.com";
+      description = ''
+          ({option}`mailserver.certificateScheme` == `acme`)
+
+        When the `acme` `certificateScheme` is selected, you can use this option
+        to override the default certificate name. This is useful if you've
+        generated a wildcard certificate, for example.
+      '';
+    };
+
     enableImap = mkOption {
       type = types.bool;
       default = true;