services/avahi: Add domainName setting

Fixes #15795.

nixos/modules/services/networking/avahi-daemon.nix

···
     use-ipv4=${if ipv4 then "yes" else "no"}
     use-ipv6=${if ipv6 then "yes" else "no"}
     ${optionalString (interfaces!=null) "allow-interfaces=${concatStringsSep "," interfaces}"}
+    ${optionalString (domainName!=null) "domain-name=${domainName}"}
 
     [wide-area]
     enable-wide-area=${if wideArea then "yes" else "no"}
···
         description = ''
           Host name advertised on the LAN. If not set, avahi will use the value
           of config.networking.hostName.
+        '';
+      };
+
+      domainName = mkOption {
+        type = types.str;
+        default = "local";
+        description = ''
+          Domain name for all advertisements.
         '';
       };