HTTP/2 with httpx backend

aiohttp does not support http/2, and has no immediate plans to: https://github.com/aio-libs/aiohttp/issues/320 https://github.com/aio-libs/aiohttp/issues/863

likewise moto does not have plans to, as werkzeug does not support it: https://github.com/getmoto/moto/issues/8904

But AWS does support it, as does httpx, so aibotocore should be able to when running with httpx as the backend. For tests it should be possible to run a local proxy server such as nginx or nghttpx to translate requests from http/2 before they're forwarded to moto.

I struggled to make progress with this in #1085, but @thehesiod noted
> it's critical when using http/2 w/ httpx to not have it pre-populate the Connection header when creating the Client

in https://github.com/aio-libs/aiobotocore/pull/1085#discussion_r2087457968