Add support for SMTP TLS reports

When enabled the tlsrpt services will send out aggregated reports about
TLS connections the local Postfix made to interested parties, who set up
a `_smtp._tls` TXT record with a rua attribute.

Introduces mailserver.systemContact to specify an administrative contact
advertised in these automated reports.

default.nix

@@ -94,6 +94,17 @@ in
       '';
     };
 
+    systemContact = mkOption {
+      type = types.str;
+      example = "postmaster@example.com";
+      description = ''
+        The email address where the administrative contact for this mail server is reachable.
+
+        Currently, this is only required when one of the following features is enabled:
+        - SMTP TLS reports (`mailserver.tlsrpt.enable`)
+      '';
+    };
+
     systemDomain = mkOption {
       type = types.str;
       default =
@@ -1032,6 +1043,8 @@ in
       };
     };
 
+    tlsrpt.enable = mkEnableOption "delivery of SMTP TLS reports according to RFC 8460";
+
     debug = {
       all = mkOption {
         type = types.bool;