Allow disabling CoreSchema validation for faster startup times

[adriangb]

Selected Reviewer: @hramezani

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[adriangb]

please review

[mrtolkien]

Just wanna say that using PYDANTIC_SKIP_VALIDATING_CORE_SCHEMAS=true combined with model_config = ConfigDict(defer_build=True) in my AWS Lambda reduced startup time from 4s to 1s.

I have a common library of Pydantic objects for multiple Lambdas, which mean a given one only uses a small subset of them despite them being imported with the library.

I'm not exactly sure what both those things do tbh, but it worked for my use-case 😬

[MarkusSintonen]

@adriangb would it be possible to change the logic here so that the env would be boolified? So we could use PYDANTIC_SKIP_VALIDATING_CORE_SCHEMAS=false to do the validation eg in CI but omit it in local tests (via pytest_env in pyproject would use PYDANTIC_SKIP_VALIDATING_CORE_SCHEMAS = "true")