Add support for PlayReady Hardware DRM

[vladimir-kazakov]

The PlayReady DRM uses 2 key systems: com.microsoft.playready (Software DRM) and com.microsoft.playready.hardware (Hardware DRM). Currently, dash.js supports only Software DRM. Trying to use Hardware DRM results in the following error in the console:

DRM: No license server URL specified! 

I assume this happens because dash.js doesn't support the com.microsoft.playready.hardware key system.

Supporting Hardware DRM is important, because movie studios more often and often require higher level of protection for higher value content. Moreover, new versions of PlayReady have features that work only when Hardware DRM is used (for example, the ability to request CBC keys in order to play cbc1 and cbcs content).

More info about how Hardware DRM must work can be found here.

[dsilhavy]

We added support for custom system strings here: #3859 . A demo is available here: https://reference.dashif.org/dash.js/nightly/samples/drm/system-string-priority.html