Add Unix Domain Socket support

[florimondmanca]

"For consideration" :-)

Closes #138

Based on work in encode/httpx#511

I think it turns out the best option is really just to add uds=... support like for local_address. UDS is just a different network transport layer than TCP, but all of HTTP still applies, and so we can reuse our connection pool, HTTPS support, HTTP/2 support, etc.

It's a bit disappointing because it'd be nice it this was easy to create as a separate transport, but a/ reusing the connection pool while customizing how the socket is established is pretty much impossible, and b/ re-implementing everything just to change a few lines in AsyncConnectionPool and AsyncHTTPConnection seems overkill (and if I had to implement this as a third-party package, I'd find it pretty daunting).

[tomchristie]

Will need to take a comprehensive review over it, but yeah I'm in favour of this.

[tomchristie]

Love it! I'm happy with this whenever you are.
Let's also doc it up alongside encode/httpx#1165

I don't think we want to push these configurations into high level API as well, it's valid for us to try to keep the surface area lower there, while still providing for some more advanced stuff.

Also there's a nice consistency in that Client(transport=...) makes it more naturally clear that you're probably also disabling auto env-based proxies from being used, because it naturally implies "This is the transport your requests will be sent over". Whereas Client(uds=...) feels much more ambiguous there, leading to... "What happens if I'm using uds=... and I've got ALL_PROXY set?".

[lundberg]

Awesome ❤️