nixos/mpd: add services.mpd.fluidsynth option

fluidsynth is compiled in but soundfont-fluid needs to be explicitely
pulled in and path configured, an option makes it much simpler to use

nixos/modules/services/audio/mpd.nix

@@ -21,6 +21,12 @@ let
 
     ${optionalString (cfg.network.listenAddress != "any") ''bind_to_address "${cfg.network.listenAddress}"''}
     ${optionalString (cfg.network.port != 6600)  ''port "${toString cfg.network.port}"''}
+    ${optionalString (cfg.fluidsynth) ''
+      decoder {
+              plugin "fluidsynth"
+              soundfont "${pkgs.soundfont-fluid}/share/soundfonts/FluidR3_GM2-2.sf2"
+      }
+    ''}
 
     ${cfg.extraConfig}
   '';
@@ -133,6 +139,14 @@ in {
           parameter is omitted from the configuration.
         '';
       };
+
+      fluidsynth = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          If set, add fluidsynth soundfont and configure the plugin.
+        '';
+      };
     };
 
   };