helper + github actions for QA?

[yarikoptic]

on push and cron those could at least verify that every annexed file is still available from our S3

I think we need to establish some helper to given a dandiset execute necessary tests + create a github action so that backups2datalad.py then should instantiate workflows invoking that action which "installs" all necessary for helper dependencies and runs the tests and reports on success/failure.

[yarikoptic]

the easiest way to check that URLs are still valid would be

git annex fsck -f web --fast -J10

but it puts strain on our dandi-api since api URL is considered first by git-annex before the direct S3 URL, so we might want to wait for being able to assign costs to urls: filed https://git-annex.branchable.com/todo/assign_costs_per_URL_or_better_repo-wide___40__regexes__41__/