Add HLS characteristics support

[dio-az]

Support HLS characteristics as defined in RFC 8216 Section 4.3.4.1 per sequence. Additionally, support FORCED subtitles and make AUTOSELECT adjustable.

Fixes #1049

Sample mapping

{
  "sequences": [
    {
      "clips": [
        {
          "type": "source",
          "path": "tears-of-steel-avc1-1500k.mp4"
        }
      ]
    },
    {
      "clips": [
        {
          "type": "source",
          "path": "tears-of-steel-aac-128k.mp4"
        }
      ],
      "language": "eng"
    },
    {
      "clips": [
        {
          "type": "source",
          "path": "tears-of-steel-en-hoh.srt"
        }
      ],
      "language": "eng",
      "label": "English (CC)",
      "characteristics": "public.accessibility.transcribes-spoken-dialog"
    },
    {
      "clips": [
        {
          "type": "source",
          "path": "tears-of-steel-en-for.srt"
        }
      ],
      "language": "eng",
      "label":"English (forced)",
      "forced": true
    }
  ]
}

Known limitations

• For characteristics such as public.easy-to-read or public.accessibility.*, the label must be explicitly specified. The default label does not take characteristics into account.