[Feature] Improved JSON Schema support

[riedgar-ms]

Add some extra support for JSON schema, to work better with the langchain examples:

• Allow definitions to be given in a definitions key, as well as $defs
• Allow for single-entry allOf blocks

[riedgar-ms]

Tag @hudson-ai for info

[Harsha-Nori]

@nopdive might have thoughts too

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.49%. Comparing base (06cc3f5) to head (ca5e477).

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #787      +/-   ##
==========================================
+ Coverage   55.51%   62.49%   +6.97%     
==========================================
  Files          55       55              
  Lines        4071     4082      +11     
==========================================
+ Hits         2260     2551     +291     
+ Misses       1811     1531     -280     

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[nopdive]

LGTM -- this really helps on supporting a wider variety of json schemas!