feat: BeforeValidators casting a simple string to `{"type": "thing"}`

[gselzer]

one "nice to have" (but definitely not mandatory) feature here is a BeforeValidator that casts a simple string to {"type": "thing"}

def _str_to_type_dict(obj: Any) -> Any:
    if isinstance(obj, str):
        return {"type": obj}
    return obj

AnyController = Annotated[..., BeforeValidator(_str_to_type_dict), Field(discriminator="type")]

then you can

Camera(controller="orbit")

but again, not mandatory, and anyone familiar with pydantic also knows they can do

Camera(controller={"type": "orbit"})

Originally posted by @tlambert03 in #42 (comment)

[gselzer]

Other classes that might benefit from this:

• ResizePolicy
• ColorModel, specifically UniformColor