Add metadata to video using FFmpeg

[GarbageHaus]

This update adds the --add_video_metadata option (default is False) to include the metadata of a generation in the final video output.

This works by:

1. Generating the video.
2. Wrapping the metadata in a JSON.
3. Checking if FFmpeg exists and appending the metadata to the "comment" field in a temp file.
4. Removing the original, renaming the temp file to the original name.

**Why?**🤔

1. key/value pairs can be added arbitrarily but there may be compatibility issues with other software which might discard this.
2. PyAV seems to lack a way of adding metadata before generating the file (I may be mistaken)
3. If you need this, I assume FFmpeg is part of the path anyways and doesn't load any 3rd party libraries that require updates to the requirements.txt

[kohya-ss]

Thank you for this! This is great. Please wait a little while until we merge the other PRs.