Add support for Certurl element

[tobbee]

The new DASH-IF Certurl element should be added if listed in the CPIX document.

[avelad]

Could this change add FairPlay support to public LiveSim streams?

eg: https://livesim2.dashif.org/livesim2/drm_EZDRM-1-key-cbcs/testpic_2s/Manifest.mpd and https://livesim2.dashif.org/livesim2/drm_EZDRM-2-keys-cbcs/testpic_2s/Manifest.mpd