feat(discovery): add support for ontology-class fields

[davidlougheed]

implements back-end support for the new ontology-class field type in the discovery config, for both querying and plots, rather than having to do everything with ontology class labels.

corresponding bento public PR: bento-platform/bento_public#355

[codecov]

Codecov Report

❌ Patch coverage is 94.91525% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.50%. Comparing base (aa54de2) to head (6a013f2).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
chord_metadata_service/discovery/stats.py	92.00%	1 Missing and 1 partial ⚠️
chord_metadata_service/discovery/fields.py	96.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #679      +/-   ##
===========================================
- Coverage    94.54%   94.50%   -0.04%     
===========================================
  Files          135      135              
  Lines         5575     5591      +16     
  Branches       536      539       +3     
===========================================
+ Hits          5271     5284      +13     
- Misses         213      215       +2     
- Partials        91       92       +1     

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