Support for dashif-Authzurl

[dsilhavy]

Is your feature request related to a problem? Please describe.
DASH-IF DASH-IF Interoperability Points; Part 6: Content Protection defines a dashif-Authzurl element:

One or more Authzurl elements may be added under the ContentProtection descriptor. It contains the URL for
an authorization server allowing to receive authorization tokens that may be required for requesting a license from a
license server. It has the optional attribute @authzType that is a string describing the authorization token type served
by this authorization server. The meaning of this attribute is specific to the server serving the token.

For DRM systems that require proof of authorization to be attached to the license request, one or more
dashif:Authzurl elements should be present under the ContentProtection descriptor, containing the default
URL to send authorization requests to.