✨ Allow specifying the `foreign_key` using an instance of `ForeignKey`

[earshinov]

~~…to let the user define additional sqlalchemy.orm.ForeignKey attributes, such as ondelete and onupdate, for foreign keys defined in a base model.~~

UPD YuriiMotov:

Currently we can specify foreign_key as an instance of sqlalchemy.ForeignKey passing it via sa_column_args:

    class Item(SQLModel, table=True):
        owner_id: Optional[int] = Field(
            default=None, sa_column_args=(ForeignKey("user.id", ondelete="SET NULL"),)
        )

But in current implementation it doesn't work if we use inheritance:

    class Base(SQLModel):
        owner_id: Optional[int] = Field(
            default=None, sa_column_args=(ForeignKey("user.id", ondelete="SET NULL"),)
        )

    class Asset(Base, table=True):
        id: Optional[int] = Field(default=None, primary_key=True)

    class Document(Base, table=True):
        id: Optional[int] = Field(default=None, primary_key=True)

The code above will lead to "This ForeignKey already has a parent" error.

This PR fixes that error and allows passing an instance of ForeignKey as a value of foreign_key parameter of Field:

    class Item(SQLModel, table=True):
        owner_id: Optional[int] = Field(
            default=None,
            foreign_key=ForeignKey("user.id", ondelete="SET NULL"),
        )