Add support for watermarking

[yairpe]

Add support for adding watermark to the output videos
It is just adding some parameters to ffmpeg so it should be fairly easy to implement.

https://ottverse.com/adding-a-watermark-in-ffmpeg-a-comprehensive-guide/

something like:
$video->hls()
->x264()
->watermark('./watermark.png', '10:10')
->addRepresentations([$r_360p, $r_480p, $r_720p])
->save();