ASGI

With the recent bug-fix in #1463, I noticed a lack of tests directly testing UnionDefinition.get_type_resolver and the function that it returns. We should add these.

Should add a section to the docs for third-party examples and guides using Mangum

It would be nice if there was a new clause which can be added to queries called callback.

For example:

def callback_handler(success, value):
    if success:
        print(f'The query was successful and returned {value}')
    else:
        print("The query failed")

Band.select().callback(callback_handler).run_sync()

The callback could be a function or coroutine, wh