support date columns

[hillalex]

Support x column values in date as well as numeric format. Try to parse as numbers; if that fails, parse as dates; if that fails, return a 400.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.67%. Comparing base (2e7d650) to head (a6096dd).
Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #6      +/-   ##
==========================================
+ Coverage   91.93%   92.67%   +0.73%     
==========================================
  Files           4        4              
  Lines         248      273      +25     
==========================================
+ Hits          228      253      +25     
  Misses         20       20              

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