Support CPIX for 3rd party managed DRM keys

[KarlGallagher]

Hey,

DASH-IF has a nice spec for packager/streamer systems to ingest DRM encryption keys from 3rd party key management systems - CPIX - https://dashif.org/docs/CPIX2.2/Cpix.html

I think it would be good if shaka-streamer supported this spec.
it should provide for greater compatibility/flexibility for DRM users
and automatically supports not only Widevine but other DRM schemes as-well

This is probably related to : #21

[joeyparrish]

This is a great idea. It depends on Shaka Packager support for CPIX, though. @kqyang, I couldn't find a feature request for CPIX on Packager, nor did I see it in the CHANGELOG. Is CPIX supported yet in Shaka Packager?

[kqyang]

@joeyparrish No, it is not supported yet.

[joeyparrish]

Thanks, KQ. We'll keep this Streamer issue on our backlog for now, then.