Add support for sending luna commands

[jonatino]

Luna commands are a lot more powerful and normally not exposed via the websocket api.

Example of use to change backlight levels:

const LUNA_SET_CONFIGS: &'static str = "com.webos.service.config/setConfigs";
const LUNA_SET_SYSTEM_SETTINGS: &'static str = "com.webos.settingsservice/setSystemSettings";
const LUNA_TURN_ON_SCREEN_SAVER: &'static str = "com.webos.service.tvpower/power/turnOnScreenSaver";
const LUNA_REBOOT_TV: &'static str = "com.webos.service.tvpower/power/reboot";
const LUNA_REBOOT_TV_WO4: &'static str = "com.webos.service.tv.power/reboot";
const LUNA_SHOW_INPUT_PICKER: &'static str = "com.webos.surfacemanager/showInputPicker";
const LUNA_SET_DEVICE_INFO: &'static str = "com.webos.service.eim/setDeviceInfo";
const LUNA_EJECT_DEVICE: &'static str = "com.webos.service.attachedstoragemanager/ejectDevice";
const LUNA_SET_TPC: &'static str = "com.webos.service.oledepl/setTemporalPeakControl";
const LUNA_SET_GSR: &'static str = "com.webos.service.oledepl/setGlobalStressReduction";
const LUNA_SET_WHITE_BALANCE: &'static str = "com.webos.service.pqcontroller/setWhiteBalance";

let category = "picture";

let params = json!(
 {
   "category": category,
   "settings": {
     "backlight": 50
   }
  }
);

client.send_luna_command(LUNA_SET_SYSTEM_SETTINGS, params).await.unwrap();

[kziemianek]

Looks good, thank you 👍🏼