Add support for two bit rates per stream: "low" 480p and "high" 1080p

[eukreign]

The SDK part of this is defined here: https://github.com/lbryio/lbry-sdk/issues/2879

• [ ] download both the high definition and low definition blobs/file for a claim
• [ ] when app requests videos to play it will pass which versions of the video should play, lbrytv should return the correct version

[anbsky]

This is currently done via transcoding.