Support validation of git-annex content available in an S3 remote

[nellh]

For OpenNeuro to host datasets with remote annexed content, we should support a validation mode that can access (or skip) that remote content as needed.

[yarikoptic]

yes! alternatively/complimentary -- could there be some generalization so we could validate some "manifest" structure which would contain e.g. list of filenames, some of them with content (.json or .tsv) and/or URLs for those files online, so could be accessed via smth like https://github.com/fsspec/ (in python - very easy).

That would allow for BIDS validation across archives where data might be on S3 or other HTTP urls, or local -- but then all going through the same interface.