Add option to download just the smaller images for training, but still record checksums of full sized originals?

> It [downsample] was intended as the smaller versions used for model training, so you get the original images for reference, processing, etc., but can also just immediately get training images.

It might be useful to give the option to keep only resized images (while still getting a checksum of the full-sized original). This could go into an issue for a future version though.

_Originally posted by @thompsonmj in https://github.com/Imageomics/cautious-robot/issues/1#issuecomment-2148298813_

Start by reading the downloader CLI and the existing downsample and checksum flow. Determine how downloaded images and full-sized originals are currently handled, then define the option's expected behavior: retain only resized training images while recording checksums for the originals. Done means the option works without losing checksum records and is covered by the relevant tests.

Written by the indexing model from the issue text.