feat: add schema_reference field to uns during add-labels

[nayib-jose-gloria]

#518

Changes:

• add schema_reference as reserved column in 'uns' for validation
• populate schema_reference field during add-labels step

[codecov]

Codecov Report

Merging #626 (7135a62) into main (ccad8d9) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #626      +/-   ##
==========================================
+ Coverage   83.02%   83.08%   +0.05%     
==========================================
  Files          19       19              
  Lines        1703     1709       +6     
==========================================
+ Hits         1414     1420       +6     
  Misses        289      289              

Flag	Coverage Δ
unittests	83.08% <100.00%> (+0.05%)	⬆️

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Files Changed	Coverage Δ
cellxgene_schema_cli/cellxgene_schema/env.py	100.00% <100.00%> (ø)
...lxgene_schema_cli/cellxgene_schema/write_labels.py	94.30% <100.00%> (+0.14%)	⬆️

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[danieljhegeman]

LGTM