Add codec arg to process_video

[LinasKo]

Description

VideoSink provides an argument for a FourCC codec, allowing encoding the video in different formats. process_video, however, does not and defaults to mp4v.

This PR adds the same arg, hardcode-defaulting to mp4v.
Note: An alternative implementation would make process_video codec have the type Optional[str] and link it with VideoSink, but this is more concise and self-documenting.

Type of change

• New feature (non-breaking change which adds functionality)

How has this change been tested, please provide a testcase or example of how you tested the change?

https://colab.research.google.com/drive/16ggPDIUnk808HE0nUG-H7QCDjTncjldC?usp=sharing

Any specific deployment considerations

⚠️ Turns out H264, which I meant to enable, is not trivial to set up.

cv2.VideoWriter_fourcc fails silently, without throwing an error, doesn't even produce the file as an output. (tested in a different Colab)

Similar issues plague others - XVID, YVYU, etc.

Leaving this as draft until we figure out what the codec behaviour is.

Docs

• Docs updated? What were the changes:

[LinasKo]

I am closing for now. We'd like greater certainty surrounding the codecs first.