Provide sanitization(s) in `{entity_plural}.tsv`

[yarikoptic]

I did not want to derail ongoing

• Alternative sanitization approach #168

but per our discussion today and a little more thinking I think we should centralize reporting of sanitizations to the files where users could potentially expect them to find. E.g. for sub-.../ses-{label} sanitizations they logically should appear in sub-.../sessions.tsv which already reserves (https://bids-specification.readthedocs.io/en/stable/modality-agnostic-files/data-summary-files.html#sessions-file) to have first column session_id (as that ses-{label}) . I think we should just complement it with the column session_id_orig.

If we expect for such sanitizations to be consistent across subjects -- we should just place such files on the top level. And that should be "ok" due to BIDS inheritance principle.
That would allow also to lookup prior down sanitizations in such a file BEFORE doing sanitiziation! This way users might even pre-populate them for us if they have better ideas!

The same then can generalize to ANY entity we sanitize. Gotchas:

• only a few entities have the corresponding {entity_plural}.tsv files already prescribed! More on a (future) attempt to harmonize that -- Formalize "life-cycle"/movement and placement of metadata bids-standard/bids-2-devel#85 . So we might need to .bidsignore some of such files, but IMHO it would only be good as would show the value!