Support ASGI lifespan state

[GuillaumeDesforges]

It would be awesome of httpx could support ASGI's lifespan state: https://asgi.readthedocs.io/en/latest/specs/lifespan.html

Scope

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state Optional(dict[Unicode string, Any]) – An empty namespace where the application can persist state to be used when handling subsequent requests. Optional; if missing the server does not support this feature.

Lifespan state allows to share state between requests, such as a ML model in GPU or a connection to a database.

It is supported by other important ASGI servers such as uvicorn (ref: Kludex/uvicorn#1818)

However httpx is the recommended library to test FastAPI apps (AsyncClient) so we can't test such applications.