Add hardware-g815 layer

layers/hardware-g815.nix

+{ pkgs, ... }:
+
+{
+  systemd.sockets.g810-led = {
+    wantedBy = [ "multi-user.target" ];
+    partOf = [ "g810-led.service" ];
+    unitConfig = {
+      Description = "Logitech keyboard led socket";
+    };
+    socketConfig = {
+      ListenStream = "/run/g810-led.socket";
+      SocketUser = "jens";
+      Accept = "yes";
+      MaxConnections = 1;
+    };
+  };
+  systemd.services."g810-led@" = {
+    after = [ "g810-led.socket" ];
+    requires = [ "g810-led.socket" ];
+    bindsTo = [ "g810-led.socket" ];
+    unitConfig = {
+      Description = "Logitech keyboard led backend";
+    };
+    serviceConfig = {
+      ExecStart = "${pkgs.g810-led}/bin/g810-led -pp";
+      StandardInput = "socket";
+    };
+  };
+}