Update server-fetch proxy implementation for httpx 0.28+

[olaservo]

Is your feature request related to a problem? Please describe.
Proxy handling was recently added to server-fetch using the deprecated proxies argument from httpx<0.28.0: #816

Describe the solution you'd like
Switch to explicit AsyncClient creation as described here: https://github.com/encode/httpx/issues/3424
That way we can update the httpx dependency to newer versions.

[fat-tire]

I think this is just a matter of changing "proxies" to "proxy" in line 75 and 119 of server.py

See: encode/httpx#2879 for the httpx change. I don't know why they broke this when they could have been backwards-compatible.

I can submit the above change as a PR, but since they created it as an incompatible change to httpx, this would break for versions before that change. Still, I do think the newest version of httpx should probably be supported (and I suppose a "smart" invocation could be added to check if proxies is valid and use that).