[SCHEMA] Allow HED has an optional column on selected files

[VisLab]

HED used to be allowed in various .tsv files including the participants.tsv, scans.tsv, sessions.tsv, and samples.tsv as well as the .tsv files in the phenotypes directory. At some point this became disallowed, and the HED in these files may not be validated. The eeg_ds003645s_hed_demo dataset in the bids examples illustrates this usage.

This PR modifies the schema to allow them again. It does not address the phenotypes directory, as I am not sure how this should be done.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.06%. Comparing base (678e02b) to head (de688e2).
⚠️ Report is 228 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2125   +/-   ##
=======================================
  Coverage   82.06%   82.06%           
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  Files          17       17           
  Lines        1533     1533           
=======================================
  Hits         1258     1258           
  Misses        275      275           

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[effigies]

I'm not sure this was disallowed so much as an oversight in schema development, that turned into dropped support when we switched from the legacy validator.

I think this patch could probably be made against the maint/1.10.0 branch as a fixup to the schema.

[effigies]

And it looks like we're not validating phenotype files at all. That should be patched, as well...

[effigies]

Done in #2126 and merged into master in 8e77449.