Create Proxy Connectors

[seanmonstar]

Add types to hyper_util::client::proxy that impl Service, wrapping another connector, and providing HTTP proxying.

There's different required behavior depending on the type of proxy, and by having each one separate, people can by then combine them with something like tower::Either.

• Create a Tunnel proxy service which does HTTP CONNECT tunneling.
• Create a Socks proxy service.

[JPDye]

I could take this on. Seems pretty simple.

[seanmonstar]

@JPDye awesome! If you want to break it into pieces, that's great too. And you're welcome to look at hyperium/hyper-util#140 as a start, or try it differently.

The final goal is that we could replace the internals in reqwest with these util types.