Support DASH SubRepresentation

Bug report

Describe the bug

Here is a clear and concise description of what the problem is:

The manifest of this stream make use of SubRepresentation fields that are not implemented in ISA,
then fallback on audio only

#KODIPROP:inputstream=inputstream.adaptive
#KODIPROP:inputstream.adaptive.manifest_type=mpd
#KODIPROP:inputstream.adaptive.stream_headers=User-Agent=RTP+Play%2F2.0.26+%28Linux%3BAndroid+11%29+ExoPlayerLib%2F2.11.8
https://streaming-ondemand.rtp.pt/nas2.share/h264/512x384/p9165/p9165_1_20210816161007e001t1185d/master.mpd?app

This is used on some RTPPlay streams
ref. @enen92 for RTPPlay addon

a future implementation will be required

MPD/M3U8s/ISM

An example or copy of a manifest (or manifests for HLS - master and variants) can be found here:
DashManifestSubRepresentation.txt

note: Once the issue is made we require you to update it with new information should that be required.
Team Kodi will consider your problem report however, we will not make any promises the problem will be solved.

Start with the attached DashManifestSubRepresentation.txt and the reported RTPPlay MPD to understand which DASH SubRepresentation fields are encountered. Locate the inputstream.adaptive DASH manifest handling for these fields; done means the affected streams no longer fall back to audio-only playback.