mpd_edit:add_subtitles

Adds an `AdaptationSet` with this structure:
    ```
    <AdaptationSet mimeType="text/vtt" lang="deu">
        <Role schemeIdUri="urn:mpeg:dash:role:2011" value="subtitle"/>
        <Representation id="sub_deu-vtt" bandwidth="1000">
            <BaseURL>tos-deu.vtt</BaseURL>
        </Representation>
    </AdaptationSet>
    ```

Example:

    {
      "tool": "mpd_edit:add_subtitles",
      "parameters": {
        "input_manifest": "dash.mpd",
        "output_manifest": "dash_new.mpd",
        "inputs": [
            {
                "base_url": "subs-en-US.vtt",
                "language": "en-US",
                "textrole": "subtitle"
            {,
            {
                "base_url": "subs-es-ES.vtt",
                "language": "es-ES",
                "textrole": "caption"
            }
        }
      }
    }