🐛 Use transport parameter in httpx.AsyncClient

[TheoBabilon]

Fixes #16

Note that it currently makes pyright unhappy with

 Argument of type "FastAPI" cannot be assigned to parameter "app" of type "_ASGIApp" in function "__init__"

but I believe this will be fixed when encode/httpx#3109 is released, so I'm not adding any # type: ignore for the sake of simplicity.

Also wondering if fastapi docs itself should be updated https://fastapi.tiangolo.com/advanced/async-tests/#example

[Kludex]

Thanks! 👍