New SubtitleOffset / PlaybackRate params on progress api

[Luke 37008]

You can now report these values to the server as part of your playback start/progress check-ins:

https://github.com/MediaBrowser/Emby/wiki/Playback-Check-ins

Additionally, when the user changes these values during playback, you can now trigger a progress report immediately, and specify either SubtitleOffsetChange or PlaybackRateChange as the EventName:

https://github.com/MediaBrowser/Emby/wiki/Playback-Check-ins#playback-progress

Apps that receive remote control commands can now support receiving the following commands: SetPlaybackRate, SetSubtitleOffset and IncrementSubtitleOffset, discussed here:

https://github.com/MediaBrowser/Emby/wiki/Web-Socket

Additionally, for mobile apps that do remote control, you can now control these values over the api, using the SetPlaybackRate, SetSubtitleOffset and IncrementSubtitleOffset commands discussed here:

https://github.com/MediaBrowser/Emby/wiki/Remote-control