Add versions to dataset schemas

[mdekstrand]

This adds versions to dataset schemas, and documents that datasets are backwards-compatible but not forwards-compatible.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.51%. Comparing base (60f6938) to head (7171e8f).
Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #721      +/-   ##
==========================================
+ Coverage   90.49%   90.51%   +0.01%     
==========================================
  Files         137      137              
  Lines        9505     9523      +18     
==========================================
+ Hits         8602     8620      +18     
  Misses        903      903              

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