Remote control of Media Session

[markafoltz]

This is a followup to #149.

An Open Screen Protocol agent may want to control media playback on another agent through common media controls (play/pause, mute/unmute, volume, seek, next/previous track etc.)

This is slightly different from remote control of a remote playback session, as the latter is focused on controlling a specific media element. Media Session allows the application to determine how to handle requests for media controls and handle cases like next/previous track which span elements.

This FR is to define an OSP extension to allow the Media Session API to be implemented by OSP.

https://w3c.github.io/mediasession/