CAT (Common Access Token) support

[tobbee]

Sync with dash.js and align with #176 Annex I for a good use case.

Dash-Industry-Forum/dash.js#4395

[Luiz-Monad]

This would be a nice to have. I'm using my own middleware handler and http headers to check that, then I have a patch on the ExoPlayer to send the headers I need. I never knew there was a way of having tokens.