Introduce HLS compatibility

[sandersaares]

It can be a bit strange to discuss HLS in DASH-IF but hey, it is important and many of the same concerns with content security also apply to HLS. For example, what's the HLS equivalent for <dashif:laurl>? There is none.

I propose we try to generalize our content security guidelines to also apply to HLS content.

We already do this implicitly, considering HLS and CMAF interactions, but we do not go as far as mentioning actual HLS manifest files. I think we should.

This might need some alignment but perhaps it would be suitable as vNext goal (for 2021 publication).

[Heronyme]

I would consider introducing a LAURL attribute to the EXT-X-KEY and EXT-X-SESSION-KEY tags.

Widevine has already introduced a non-standard KEYID attribute. So, it might be OK to add something like this. Probably using a separate attribute would be better than somehow trying to cram it in the existing URI attribute (which logically sounds like it could store an LA URL), because the URI is already populated in some DRM specific ways.