uds support in proxy classes

[NewUserHa]

related to #857
currently, AsyncConnectionPool supported uds but the proxy classes(socksproxy) inherited from it does not

[lovelydinosaur]

Is making a unix domain socket connection to a proxy a use-case anyone's bumped up against yet?

[NewUserHa]

It probably is an edge use case, but with the current httpcore design it is very simple to support.