Support 'sni_hostname' extension with SOCKS proxy.

[Allgot]

• Initially raised as discussion Support for `sni_hostname` extension when using SOCKS proxy. #771

Currently, there is an issue with sni_hostname not functioning correctly when the connection involves a socks proxy.
This problem arises due to the oversight in commit #696, where the update to _sync/socks_proxy.py and _async/socks_proxy.py was omitted.

Code for reproduction (written by @karosis88 at #771)

Set up proxy server (defaults to 1080 port)

git clone https://github.com/itsjfx/python-socks5-server
python python-socks5-server/server.py

Client

import httpcore

pool = httpcore.SOCKSProxy("socks5://127.0.0.1:1080")
response = pool.request("GET", 
                        "https://185.199.108.153:443", 
                        headers=[(b'Host', 'www.encode.io')], 
                        extensions={'sni_hostname': 'www.encode.io'})
print(response)

Expected output

<Response [200]>

Actual output

httpcore.ConnectError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: IP address mismatch, certificate is not valid for '185.199.108.153'. (_ssl.c:1123)

Suggested fix

Update the _sync/socks_proxy.py and _async/socks_proxy.py in a manner similar to that of the _sync/connection.py and _async/connection.py.

Example fix for _async/socks_proxy.py:

218    218      timeouts = request.extensions.get("timeout", {})
       219   +  sni_hostname = request.extensions.get("sni_hostname", None)
219    220      timeout = timeouts.get("connect", None)

261          -  "server_hostname": self._remote_origin.host.decode("ascii"),
       262   +  "server_hostname": sni_hostname
       263   +  or self._origin.host.decode("ascii"),

[lovelydinosaur]

Thanks for this, I'll retitle it so that it's more specific.

[lovelydinosaur]

@Allgot Would you like to issue a pull request resolving this?

[Allgot]

@tomchristie
No problem!

But since this is my first contribution to this repository(in terms of pull requests), my request might not be perfect. It would be great if you can correct errors, if exist :)

[Allgot]

I've send a PR. Please refer to #774