support optional annotated field with default value

[proever]

Currently, it is not possible to specify a SQLModel with an Optional complex type, such as a Decimal, as follows:

from decimal import Decimal
from typing import Annotated
from sqlmodel import Field, SQLModel


class PriceModel(SQLModel, table=True):
    id: int | None = Field(primary_key=True, default=None)
    price: Annotated[Decimal, Field(decimal_places=2, max_digits=9)] | None = None

Doing so results in the following error (see #67, #312) in get_sqlalchemy_type:

TypeError: issubclass() arg 1 must be a class

This PR attempts to fix this issue by adding a check in get_sqlalchemy_type for whether the type of the Field is a typing_extensions.AnnotatedAlias. If it is, instead of using the class of the AnnotatedAlias for the following comparisons (which results in the above error), it uses AnnotatedAlias.__origin__. Similarly, it infers the metadata from AnnotatedAlias.__metadata__ instead of the AnnotatedAlias itself.

In my testing, this approach seems to work well. I also added a simple test here that checks whether restrictions on an optional annotated field are enforced on the database side. It could probably be improved or extended.

[proever]

looks like some (many) tests are failing, I'll try to fix them!

Is this feature something that should work with Pydantic v1 too? I'm not too familiar with it.