Add support for generator factories

[cfxegbert]

I want to do something like the following:

@service
def create_transaction() -> Iterator[Transaction]:
    transaction = Transaction()
    try:
        yield transaction
    finally:
        transaction.commit()

and also an async version:

@service
async def create_transaction() -> AsyncIterator[Transaction]:
    transaction = Transaction()
    try:
        yield transaction
    finally:
        await transaction.commit()