Merge video stream code into core unity runtime

[sorenhan]

Currently the video streaming code is exposed as an optional extension, in part because some logic for specific sites (youtube, mixer etc) is not generalized enough for inclusion.

Video playing may be so generic these days that we may want to change this. We may want to do these steps

1. simplify the video streaming interface to expose raw streams only
2. move the video player code in as part of the core runtime
3. move the site-specific logic into a custom MRE.

[sorenhan]

Having discussed this interally, I think we're recommending leaving it as an extension, but provide a general-purpose implementation in the testbed. LibVLCSharp looks like a great candidate.

The benefit would be that the minimal MRE host apps could stay tiny if they don't need this, or can still use their own video player if they so desire, but the easiest solution would be to just take the provided solution.

The same pattern could be used for browser implementations, too (#602 )