nixos/wireless: add extraConfig section to networking.wireless

This allows the user to add `wpa_supplicant` config options not yet supported by Nix without having to write the entire `wpa_supplicant.conf` file manually.

nixos/modules/services/networking/wpa_supplicant.nix

@@ -8,6 +8,7 @@ let
     ${optionalString cfg.userControlled.enable ''
       ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=${cfg.userControlled.group}
       update_config=1''}
+    ${cfg.extraConfig}
     ${concatStringsSep "\n" (mapAttrsToList (ssid: config: with config; let
       key = if psk != null
         then ''"${psk}"''
@@ -165,6 +166,17 @@ in {
           description = "Members of this group can control wpa_supplicant.";
         };
       };
+      extraConfig = mkOption {
+        type = types.str;
+        default = "";
+        example = ''
+          p2p_disabled=1
+        '';
+        description = ''
+          Extra lines appended to the configuration file.
+          See wpa_supplicant.conf(5) for available options.
+        '';
+      };
     };
   };