Make __signature__ a lazy property, do not deepcopy defaults

[Viicos]

Also follow stdlib dataclasses for signature parameter defaults. If a default factory is used, the placeholder '' is used.

Related issue number

Fixes #10771, #10656

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

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[codspeed]

CodSpeed Performance Report

Merging #10818 will improve performances by 11.23%

_{Comparing model-signature (fda5e91) with main (8bad227)}

Summary

⚡ 9 improvements
✅ 35 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	model-signature	Change
⚡	test_model_validators_serializers	1.1 ms	1.1 ms	+5.76%
⚡	test_nested_model_schema_generation	1.4 ms	1.4 ms	+7.13%
⚡	test_recursive_model_schema_generation	1.4 ms	1.3 ms	+7.45%
⚡	test_simple_model_schema_generation	1,042.9 µs	938.1 µs	+11.17%
⚡	test_tagged_union_with_callable_discriminator_schema_generation	2.1 ms	2 ms	+5.22%
⚡	test_tagged_union_with_str_discriminator_schema_generation	2 ms	1.9 ms	+5.47%
⚡	test_deeply_nested_recursive_model_schema_generation	1.7 ms	1.6 ms	+5.72%
⚡	test_generic_recursive_model_schema_generation	1.2 ms	1.1 ms	+9.03%
⚡	test_simple_recursive_model_schema_generation	1,003 µs	901.8 µs	+11.23%

[sydney-runkle]

This is promising - are there more class attributes for which we could do this? Gives us a nice boost on schema builds :)

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
pydantic/_internal
_model_construction.py
_signature.py
_utils.py
Project Total

This report was generated by python-coverage-comment-action

[sydney-runkle]

Very impressed with this implementation, good find.

Feel free to merge at your leisure.