WIP: SOCKS proxy support

[yeraydiazdiaz]

Adds SOCKS4 proxy support via socksio 🎉

You can test this either locally using Dante via Docker as we do in socksio or simply running an ssh -vND <PORT> <OTHER_MACHINE>, then running the following script, assuming the proxy is running in localhost:1080:

import asyncio
import httpcore

async def read_body(stream):
    try:
        body = []
        async for chunk in stream:
            body.append(chunk)
        return b"".join(body)
    finally:
        await stream.aclose()

async def socks_proxy(proxy_server, proxy_mode):
    async with httpcore.AsyncHTTPProxy(proxy_server, proxy_mode=proxy_mode) as http:
        method = b"GET"
        url = (b"http", b"93.184.216.34", 80, b"/")
        headers = [(b"host", b"example.org")]
        http_version, status_code, reason, headers, stream = await http.request(
            method, url, headers
        )
        body = await read_body(stream)
        print(body)

        assert http_version == b"HTTP/1.1"
        assert status_code == 200
        assert reason == b"OK"

def main():
    proxy_server = (b'http', b'localhost', 1080)
    proxy_mode = 'SOCKS4'
    asyncio.run(socks_proxy(proxy_server, proxy_mode))

if __name__ == '__main__':
    main()

Before I carry on with SOCKS4A and SOCKS5 (or possibly I should open a different PR?) I'd like some feedback.

[yeraydiazdiaz]

This is required by SOCKS4.

SOCKS5 will require username/password and likely defining acceptable authentication methods. Which makes me think we might want separate AsyncSOCKS4Connection and AsyncSOCKS5Connection.

[yeraydiazdiaz]

I realize there's a fair bit of cleanup noise as well, sorry about that. Hopefully the important bits are clear.

[tomchristie]

Looks like there's now a merge conflict between this and master, that'd need resolving.

[yeraydiazdiaz]

Yep, this branch predates the recent changes regarding proxies.

Will update it soon.

[yeraydiazdiaz]

I think I'll hold off on this until we've decided if proxy modes are something we want at all.

encode/httpx#927 (comment) and encode/httpx#927 (comment)

[tomchristie]

SOCKS5 is now supported in 0.14.5 - https://www.encode.io/httpcore/proxies/#socks-proxy-support

(Not sure what we'd want to do about socks4, socks4a)