feat: Create validation context classes from schema.meta.context

[effigies]

Starting to think about how to write a Python validator that builds from the schema, rather than replicating the schema. Here's some bits that make it easy to generate structured classes for the validation context (schema.meta.context.context) based on either the bidsschematools schema or a schema loaded from JSON.

So far I have attrs classes that populate the context. It's unclear if it's worth doing something similar with the whole schema, which would be much more involved (patternProperties, additionalProperties, and the like).

[codecov]

Codecov Report

Attention: Patch coverage is 56.92308% with 84 lines in your changes missing coverage. Please review.

Project coverage is 74.87%. Comparing base (58732d1) to head (21cf04c).

Files with missing lines	Patch %	Lines
src/bids_validator/bidsignore.py	37.28%	35 Missing and 2 partials ⚠️
src/bids_validator/types/files.py	51.42%	34 Missing ⚠️
src/bids_validator/context_generator.py	79.03%	9 Missing and 4 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main       #2       +/-   ##
===========================================
- Coverage   93.19%   74.87%   -18.33%     
===========================================
  Files           3        7        +4     
  Lines         191      386      +195     
  Branches       55      101       +46     
===========================================
+ Hits          178      289      +111     
- Misses          5       83       +78     
- Partials        8       14        +6     

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