Add implementation of AsyncSession.run_sync()

[jnewbery]

Currently, sqlmodel.ext.asyncio.session.AsyncSession doesn't implement run_sync(), which means that any call to run_sync() on a sqlmodel AsyncSession will be dispatched to the parent
sqlalchemy.ext.asyncio.AsyncSession.

The first argument to sqlalchemy's AsyncSession.run_sync() is a callable whose first argument is a sqlalchemy.orm.Session object. If we're using this in a repo that uses sqlmodel, we'll actually be passing a callable whose first argument is a
sqlmodel.orm.session.Session.

In practice this works fine - because sqlmodel.orm.session.Session is derived from sqlalchemy.orm.Session, the implementation of sqlalchemy.ext.asyncio.AsyncSession.run_sync() can use the sqlmodel Session object in place of the sqlalchemy Session object. However, static analysers will complain that the argument to run_sync() is of the wrong type. For example, here's a warning from pyright:

Pyright: Error: Argument of type "(session: Session, id: UUID) -> int" cannot be assigned to parameter "fn" of type "(Session, **_P@run_sync) -> _T@run_sync" in function "run_sync"
  Type "(session: Session, id: UUID) -> int" is not assignable to type "(Session, id: UUID) -> int"
    Parameter 1: type "Session" is incompatible with type "Session"
      "sqlalchemy.orm.session.Session" is not assignable to "sqlmodel.orm.session.Session" [reportArgumentType]

This commit implements a run_sync() method on
sqlmodel.ext.asyncio.session.AsyncSession, which casts the callable to the correct type before dispatching it to the base class. This satisfies the static type checks.