Create `roll_call` module

This would have a `RollCall` class that would include [`check_existing_images`](https://github.com/Imageomics/cautious-robot/blob/a42a3c7f72b78bb5427d7d678e51dd102cd6b8b7/src/cautiousrobot/utils.py#L87) and the other pre-download verification functions (pulled out in #56).

- [x] Add hash-based `check_existing_images` for when `--verifier-col` is passed. This should raise an issue when there are mismatched filename and hash values to the provided CSV (i.e., a BuddyCheck prior to download).
- [x] TODO (decision needed): Should we have a flag for non-unique checksums? We could just check for uniqueness of checksums within a directory and have a final duplicate check come out of the post-download BuddyCheck as a note, here are duplicate images you have. Might be better to have it check for duplicates in the validation column ahead of time and just allow a flag `--ignore-duplicates` to allow for downloading duplicated images under different filenames.