Forward uds, local_address and retries to proxied connections

[mbeijen]

When a proxy was configured, HTTPTransport and AsyncHTTPTransport
silently dropped uds, local_address and retries instead of forwarding
them to the underlying pool.

The gap ran deeper than the transport layer:

• httpcore2.HTTPProxy accepted all three but its create_connection never
  handed them (or socket_options) to the proxy connections it built.
• ConnectionPool(proxy=...) had the same gap in all three of its proxy
  branches.
• SOCKSProxy accepted retries while the Socks5 connection had no retry
  logic at all.

Changes

• Thread retries, local_address and uds through the forward and tunnel
  proxy connections down to the inner HTTPConnection.
• Implement connect retries for Socks5 connections, mirroring the backoff
  behaviour of HTTPConnection (same exception set, same
  exponential_backoff sequence, same Trace("retry", ...)).
• Forward the options from both transports. SOCKS proxies support retries
  only; uds, local_address and socket_options are not available there,
  now noted in the transports documentation.
• Drop the local_address and uds entries from the SOCKSProxy docstring,
  which documented parameters the constructor does not accept.

Note that socket_options remains silently ignored for SOCKS proxies rather
than raising — that would be a behaviour change for anyone currently passing
the combination, so it is documented instead.

See #1160

[codspeed-hq]

Merging this PR will not alter performance

✅ 17 untouched benchmarks
⏩ 7 skipped benchmarks¹

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_{Comparing mbeijen:proxy-transport-options (de2e49b) with main (9fdafff)}

Footnotes

1. 7 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[cubic-dev-ai]

No issues found across 15 files

_{Re-trigger cubic}