Add Cancellation Tokens to Hudl.FFmpeg

[Casey-Bateman]

This adds CancellationToken support to ffmpeg renders. There are instances where a user may want to pass in a cancellation token into Hudl.FFmpeg to control the cancellation of a task based on some external force requiring cancellation. Simply put, this adds a cancellation token as the source for cancelling and killing the ffmpeg process when a task has been cancelled.

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