🏷️ Allow Discriminator for discriminator in Field

[YuriiMotov]

This PR updates type hints for discriminator parameter of Field to be in line with Pydantic's version:

    discriminator: str | types.Discriminator | None = _Unset,

Also, added tests for callable discriminator.

---

The following code works on master:

from collections.abc import Hashable
from typing import Annotated, Any, Literal, Union

from pydantic import Discriminator, Tag
from sqlmodel import Field, SQLModel

class Pie(SQLModel):
    time_to_cook: int
    num_ingredients: int

class ApplePie(Pie):
    fruit: Literal["apple"] = "apple"

class PumpkinPie(Pie):
    filling: Literal["pumpkin"] = "pumpkin"

def get_discriminator_value(v: Any) -> Hashable:
    if isinstance(v, dict):
        return v.get("fruit", v.get("filling"))
    return getattr(v, "fruit", getattr(v, "filling", None))

    class ThanksgivingDinner(SQLModel):
        dessert: Union[
            Annotated[ApplePie, Tag("apple")],
            Annotated[PumpkinPie, Tag("pumpkin")],
        ] = Field(
            discriminator=Discriminator(get_discriminator_value),
        )

apple_variation = ThanksgivingDinner.model_validate(
    {"dessert": {"fruit": "apple", "time_to_cook": 60, "num_ingredients": 8}}
)

pumpkin_variation = ThanksgivingDinner.model_validate(
    {"dessert": {"filling": "pumpkin", "time_to_cook": 40, "num_ingredients": 6}}
)

print(type(apple_variation.dessert))
# <class '__main__.ApplePie'>

print(type(pumpkin_variation.dessert))
# <class '__main__.PumpkinPie'>

.. but mypy argues with:

error: No overload variant of "Field" matches argument types "EllipsisType", "Discriminator"  [call-overload]

This PR fixes this