feat(DASH): Add Label element.

[sr1990]

This PR is related to #881

../packager \
  'in=Sintel-1280x720.mp4,stream=video,init_segment=h264_1280p/init.mp4,segment_template=h264_1280p/$Number$.m4s' \
  'in=Sintel-audio-english.mp4,stream=audio,init_segment=audioSintelEnglish/init.mp4,segment_template=audioSintelEnglish/$Number$.m4s,dash_label=English' \
  'in=Sintel-audio-hindi.mp4,stream=audio,init_segment=audioSintelHindi/init.mp4,segment_template=audioSintelHindi/$Number$.m4s,dash_label=Hindi' \
  --mpd_output h264_label.mpd --generate_static_live_mpd 

generates

<?xml version="1.0" encoding="UTF-8"?>
<!--Generated with https://github.com/shaka-project/shaka-packager version d5ca6e84e6-debug-->
<MPD xmlns="urn:mpeg:dash:schema:mpd:2011" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="urn:mpeg:dash:schema:mpd:2011 DASH-MPD.xsd" profiles="urn:mpeg:dash:profile:isoff-live:2011" minBufferTime="PT2S" type="static" mediaPresentationDuration="PT52.20833206176758S">
  <Period id="0">
    <AdaptationSet id="0" contentType="video" width="1280" height="720" frameRate="96/4" segmentAlignment="true" par="16:9">
      <Representation id="0" bandwidth="2942370" codecs="avc1.4d401f" mimeType="video/mp4" sar="1:1">
        <SegmentTemplate timescale="96" initialization="h264_1280p/init.mp4" media="h264_1280p/$Number$.m4s" startNumber="1">
          <SegmentTimeline>
            <S t="0" d="576" r="7"/>
            <S t="4608" d="404"/>
          </SegmentTimeline>
        </SegmentTemplate>
      </Representation>
    </AdaptationSet>
    <AdaptationSet id="1" contentType="audio" segmentAlignment="true">
      <Label>Hindi</Label>
      <Representation id="1" bandwidth="131516" codecs="mp4a.40.2" mimeType="audio/mp4" audioSamplingRate="48000">
        <AudioChannelConfiguration schemeIdUri="urn:mpeg:dash:23003:3:audio_channel_configuration:2011" value="2"/>
        <SegmentTemplate timescale="48000" initialization="audioSintelHindi/init.mp4" media="audioSintelHindi/$Number$.m4s" startNumber="1">
          <SegmentTimeline>
            <S t="0" d="288768"/>
            <S t="288768" d="287744" r="2"/>
            <S t="1152000" d="288768"/>
            <S t="1440768" d="287744" r="2"/>
            <S t="2304000" d="167936"/>
          </SegmentTimeline>
        </SegmentTemplate>
      </Representation>
    </AdaptationSet>
    <AdaptationSet id="2" contentType="audio" segmentAlignment="true">
      <Label>English</Label>
      <Representation id="2" bandwidth="131516" codecs="mp4a.40.2" mimeType="audio/mp4" audioSamplingRate="48000">
        <AudioChannelConfiguration schemeIdUri="urn:mpeg:dash:23003:3:audio_channel_configuration:2011" value="2"/>
        <SegmentTemplate timescale="48000" initialization="audioSintelEnglish/init.mp4" media="audioSintelEnglish/$Number$.m4s" startNumber="1">
          <SegmentTimeline>
            <S t="0" d="288768"/>
            <S t="288768" d="287744" r="2"/>
            <S t="1152000" d="288768"/>
            <S t="1440768" d="287744" r="2"/>
            <S t="2304000" d="167936"/>
          </SegmentTimeline>
        </SegmentTemplate>
      </Representation>
    </AdaptationSet>
  </Period>
</MPD>

[github-actions]

Closing due to inactivity. If the author would like to continue this PR, they can reopen it (preferred) or start a new one (if needed).

[joeyparrish]

Reopening. We should be able to finish this one.

[joeyparrish]

Looks good otherwise