Minor refactoring -- no behavioral changes

[sydney-runkle]

Change Summary

1. Convert some classmethods to staticmethods on the FieldInfo class, as we aren't subclassing FieldInfo. See motivation for this refactor here: Update to pyright==1.1.335 #8075
2. Cleaning up ClassVar mypy support (var naming, docstrings, grammar)
3. Move shared signature generation logic to the _signature.py file in the _internal folder. Also removed unnecessary wrapper functions for the model and dataclass signature generation.

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @samuelcolvin

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View logs

[codspeed-hq]

CodSpeed Performance Report

Merging #8236 will not alter performance

_{Comparing slight-refactoring (b29e74f) with main (22e6444)}

Summary

✅ 10 untouched benchmarks

[sydney-runkle]

Please review

[adriangb]

Looks good to me!