Add API to set arbitrary playback speed

[PierfrancescoSoffritti]

The API should be something like setPlaybackRate(playbackRate: Float), and should match whatever the IFrame Player API is doing.

See #1073 .

[santiwanti]

@PierfrancescoSoffritti let me know if you have thought of how to set up the API, but don't have time to implement it and I will happily do it. Also let me know if you need help bouncing off ideas.

[rafaelekol]

When is the next Release?

[PierfrancescoSoffritti]

Hopefully before the new year.