Add frame rate to VideoEncoder API

[Dan-Flores]

This PR adds output frame_rate to the API.

Previously, we always encoded videos using the same frame_rate as the input frames.

Now, we can set the output frame_rate when calling to_file/to_tensor/to_filelike (as in AudioEncoder), to adjusts the encoded video's frame durations. If no output frame_rate is specified, fallback to reusing the input frame_rate.

Testing:

To test, I added test_frame_rate_parameter which slows and speeds up a test video and ensures the video metadata is updated correctly. I also observed the video plays back slower or faster.

API after changes:

The complete API now looks as follows:

  encoder = VideoEncoder(frames=frames, frame_rate=25)
  encoder.to_file(
      dest="video.mp4",
      codec="libx264",
      pixel_format="yuv420p",
      crf=20,
      preset="medium",
      extra_options={
		"tune": "fastdecode",		
		"x264-params": "deblock=-1,-1:aq-mode=2
	  },
      frame_rate=25,
  )

[Dan-Flores]

I'm happy to bikeshed on parameter order. sample_rate is the last param in AudioEncoder, but that constructor is much smaller. Here, having it last might make frame_rate less discoverable?

[scotts]

My views:

1. Optional parameters should always be keyword-only.
2. codec should be the first keyword-only parameter, as it's likely to be the most used.
3. extra_options should be the last keyword-only parameter, as "catch-all" parameters tend to be last.

I don't have a preference for the order of the rest. 🚲 🛖 !