Add a convenience type to override JSON schema

[dmontagu]

Closes #5096
Closes #4079

Introduces an annotation WithJsonSchema that you can use to override the JSON schema for a type.

Also makes it so that if you raise pydantic_core.PydanticOmit while generating the JSON schema for a field of any type that has fields (i.e., model, dataclass, typeddict), that field is omitted from the generated JSON schema.

The tests added to this PR demonstrate how to use this functionality to address:

• Callable, related to JSON schema: Callable #5096
• Arbitrary types, related to model.schema() fails when using arbitrary types #4079

Selected Reviewer: @samuelcolvin

[dmontagu]

@adriangb has suggested using pydantic_core.PydanticOmit rather than a new exception class. As long as we document it somewhere (which we need to do anyway), I think that's fine, I'll make the change.

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[dmontagu]

Also Adrian I know I suggested we should re-export pydantic_core.PydanticOmit but I take it back — considering this will be pretty niche anyway I think it's fine to just leave as an import from pydantic_core unless we get feedback to the contrary. (Happy to change that though if you or Samuel prefer.)

[dmontagu]

please review

[adriangb]

Looks good to me!