Built-in video thumbnail generator (for remote video thumbnails without needing "Cache remote media" enabled)

Created by: ShittyKopper

when you disable "cache external media", video thumbnails off of remote instances do not get generated. misskey has a videoThumbnailGenerator config option to point to an external service to make that happen, but they do not provide any kind of implementation (or any documentation beyond a comment on the config file)

this provides a video thumbnail generator that uses the same thumbnail generation code path used in local files, providing a quick and dirty solution to instances that want video thumbnails without the need to store external video files permanently

video thumbnails are cached on disk under a new ./cache folder. it is safe to wipe the contents of that folder as the files in it will be re-generated on demand. there is no object storage support for video thumbnails yet (and considering it's just a cache i'm unsure if it'll be needed)

a new config option named enableBuiltinVideoThumbnailGenerator is used to enable it. it's disabled by default as it's unnecessary if caching of remote media is enabled. videoThumbnailGenerator takes precedence over it if enabled.