Add recursive option to videos_to_poses

[cleong110]

Add an option (perhaps "-r" "--recursive" to recursively search for video files within a directory. Should be easy for me to implement this with pathlib and rglob, I have a prototype version implemented already, just need to find it and do a PR

[cleong110]

Things to consider:

• I'll need to update the "missing files" logic. Currently it looks within a single dir for .mp4 files and .pose files, and compares the list. For recursion to work we need to generate these lists differently.
• While we're at it, maybe we can change it so you can change whether it looks for .mp4 files or .webm files?
• ...but supporting .webm might not work right out of the box, though, looks like you can get negative frame counts? Not being able to proper open a webm video opencv/opencv#22571 This would match my experience trying to get ffmpeg to convert Sem-Lex .webm videos to .mp4. I got some videos that I ran this utility on, then when fed to SignCLIP for embedding had "negative dimension errors"