Using different proxies in different requests of the same AsyncClient

[mxrch]

Hi !
I started adding this feature on my fork here : mxrch@7512166
I think it's all good, I added the proxy_map, ._mounts etc. to the Request object too, and I also moved the init_proxy_transport and init_transport functions to the utils file.
But I got a circular import, because I think my structure is bad and doesn't match your project structuration.

I'm using the test.py at the root to test the feature, and uninstalling the httpx library so it takes the code in the httpx folder.
Do you think you can take a look at this ? 😄

I can add you in the repo maintainers so you can make edits on my fork, just ask me !