More structured response for /now-playing endpoint

[jjst]

Currently it returns this: https://github.com/jjst/now-playing/blob/main/api/openapi/spec.yaml#L186

I'm thinking something more akin to:

{
  "metadata": {
    "success": true,
    "aggregator": {
      "module_name": "",
      "params" {}
  },
  "item": {
    "type": "song",
    "title: "The Final Countdown",
    "artist": "Europe",
    "start_time": "",
    "end_time": ""
  }
}

or for a programme

{
  "metadata": {
    "success": true,
    "aggregator": {
      "module_name": "",
      "params" {}
  },
  "item": {
    "type": "programme",
    "name: "La Marche De L'Histoire",
    "start_time": "",
    "end_time": ""
    "episode_name": "Le Congrès d'Epinay du PS il y a un demi-siècle" 
    "episode_number": 966
  }
}

start_time and end_time optional fields, full dates with timezone.

[jjst]

Structure should be compatible with #25

[jjst]

maybe item should be an array of items

[jjst]

metadata is useful especially if multiple aggregators are configured

[jjst]

Mostly implemented in #111 - missing the metadata field which i still think would be useful