Make frame selection from a video more intelligent

[danylomm]

From my understanding, right now calling ns-process-data video just randomly samples --num-frames-target images from the video. This is suboptimal because of two reasons:

1. The random strategy is super naive, maybe you could add some algorithm to select key frames based on some difference metric between images?
2. An option to fix seed for random sampling would make processing videos with nerfstudio much more reproducible