Apply connect timeout to SOCKS5 handshakes

[lin-hongkuan]

Fixes #1089.

Summary

• pass the request connect timeout into SOCKS5 handshake setup
• apply it to auth method, username/password, and connect request read/write calls
• add sync/async regression tests that assert SOCKS handshake I/O receives the connect timeout

Tests

• py -m pytest tests\_sync\test_socks_proxy.py::test_socks5_request_uses_connect_timeout_for_handshake -q
• py -m pytest tests\_async\test_socks_proxy.py::test_socks5_request_uses_connect_timeout_for_handshake -q
• py -m pytest tests\_sync\test_socks_proxy.py tests\_async\test_socks_proxy.py -q
• uvx ruff==0.5.0 format httpcore tests --diff
• uvx ruff==0.5.0 check httpcore tests
• python scripts\unasync.py --check
• git diff --check HEAD~1..HEAD

[mbeijen]

This is a duplicate of #1017 and also of #1055. Please note that httpcore is not getting active maintenance anymore, unfortunately. There is now httpx2, which is a maintained version, but that has the same problem wrt the proxy timeout!! See pydantic/httpx2#1009