refactor model serialization

[davidhewitt]

Change Summary

This refactors model serialization without behaviour changes so that more code is wrapped up in reusable generics. This should help in the longer run with implementing changes like polymorphic model serialization.

I want to follow up with similar to for dataclass serialization and fields, which might require a few iterations.

Related issue number

N/A

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with this pydantic-core (except for expected changes)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[codspeed-hq]

CodSpeed Performance Report

Merging #1855 will not alter performance

_{Comparing dh/refactor-model-serialization (7663009) with main (9822977)}

Summary

✅ 163 untouched

[davidhewitt]

I'm going to merge this and proceed with the next step of refactoring.