Support async sessions in strawberry_sqlalchemy_mapper and deprecate sync.
I left a note in the loader's __init__, but to duplicate that here:
Making blocking database calls from within an async function (the resolver) has
catastrophic performance implications. Not only will all resolvers be effectively
serialized, any other coroutines waiting on the event loop (e.g. concurrent requests
in a web server), will be blocked as well, grinding your entire service to a halt.
There's no reason for us to support a foot bazooka, hence the deprecation.