Add config.defer_build to optionally make model building lazy

[samuelcolvin]

Change Summary

This proposes a new config flag defer_model_build, when true this means the core schema, schema validator and schema serializer are not constructed for a model when it's defined, instead they're only built when model_rebuild() is called, or when the model is first called.

Two reasons this can be useful:

• so models which are only ever used nested or within TypeAdapters don't need all this logic calling - see Very slow (FastAPI) application startup after using V2 due to _core_utils.py:walk #6768 and Pydantic v2 significantly slower than v1 #6748
• so that the type namespace used when constructing models can be customised, fixes Incorrect ForwardRef class lookup in inherited models #6742

Related issue number

fix #6742

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 - somewhat, I'm going to rewrite the docs for ConfigDict anyway, so I'll add more logic then
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @adriangb

[samuelcolvin]

please review.

[adriangb]

Having this as a config flag sounds reasonable to me. For the reasons discussed in #6768 I don’t think it’s good as a default, but for large apps it seems like a reasonable trade-off.

Shouldn’t we make TypeAdapter lazy as well?

[samuelcolvin]

Let's rename to defer_build and apply to TypeAdapter too.

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[hramezani]

I think you should remove it