feat(python): Custom retrievers for external references

[Stranger6667]

Resolves #372

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 80 lines in your changes missing coverage. Please review.

Project coverage is 87.45%. Comparing base (3c25034) to head (9c76735).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
crates/jsonschema-py/src/lib.rs	0.00%	80 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #652      +/-   ##
==========================================
- Coverage   87.95%   87.45%   -0.50%     
==========================================
  Files          90       90              
  Lines       13306    13382      +76     
==========================================
  Hits        11703    11703              
- Misses       1603     1679      +76     

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[Stranger6667]

cc @jpmckinney I think it should be enough for your use case, right? If not, please, let me know and I'll adjust the behavior in a follow-up PR.

[codspeed-hq]

CodSpeed Performance Report

Merging #652 will not alter performance

_{Comparing dd/py-retrievers (9c76735) with master (3c25034)}

Summary

✅ 45 untouched benchmarks

[jpmckinney]

That was fast! Yes, I can use that both to control which URI schemes are supported, and I can use it like a registry (mapping individual URLs to specific values).