feat(DASH): Add support for "Spoken Subtitles" in tva:metadata:cs:AudioPurpose

[matvp91]

Today we solely map visually impaired and hard hearing. I've added Spoken Subtitles as a new accessibility purpose.

For reference:

<?xml version="1.0" encoding="UTF-8"?>
<ClassificationScheme uri="urn:tva:metadata:cs:AudioPurposeCS:2007">
  <Term termID="1">
    <Name xml:lang="en">Audio description for visually impaired people</Name>
  </Term>
  <Term termID="2">
    <Name xml:lang="en">Audio description for hard of hearing people</Name>
  </Term>
  <Term termID="3">
    <Name xml:lang="en">Supplemental commentary</Name>
  </Term>
  <Term termID="4">
    <Name xml:lang="en">Director's commentary</Name>
  </Term>
  <Term termID="5">
    <Name xml:lang="en">Educational notes</Name>
  </Term>
  <Term termID="6">
    <Name xml:lang="en">Main programme audio</Name>
  </Term>
  <Term termID="7">
    <Name xml:lang="en">Clean feed - no effects mix</Name>
  </Term>
  <Term termID="8">
    <Name xml:lang="en">Dialogue Enhancement</Name>
  </Term>
  <Term termID="9">
    <Name xml:lang="en">Spoken Subtitles</Name>
  </Term>
</ClassificationScheme>

[shaka-bot]

Incremental code coverage: 25.00%