riak: added extraAdvancedConfig option to service module

nixos/modules/services/databases/riak.nix

···
         '';
       };
 
+      extraAdvancedConfig = mkOption {
+        type = types.lines;
+        default = "";
+        description = ''
+          Additional text to be appended to <filename>advanced.config</filename>.
+        '';
+      };
+
     };
 
   };
···
       platform_data_dir = ${cfg.dataDir}
 
       ${cfg.extraConfig}
+    '';
+
+    environment.etc."riak/advanced.config".text = ''
+      ${cfg.extraAdvancedConfig}
     '';
 
     users.extraUsers.riak = {