feat(dash): Parse ClearKey license URL in MPD

[sr1990]

Hi,
This PR is related to parsing Clear Key license as per DASH-IF-IOP-Part6-v5.0.0. - Part 8.

Example content protection tags parsed:

          
<ContentProtection schemeIdUri="urn:mpeg:dash:mp4protection:2011" value="cenc" cenc:default_KID="<default key id>" />
            <ContentProtection value="ClearKey1.0" schemeIdUri="urn:uuid:e2719d58-a985-b3c9-781a-b030af78d30e">
                <clearkey:Laurl Lic_type="EME-1.0">License Url</clearkey:Laurl>

The player parses the default key id and license url,
and sends a POST request to license url as per https://w3c.github.io/encrypted-media/#clear-key-request-format to retrieve the decryption key/s.