Add TCP keepalive socket options

[hownowstephen]

Summary

• Configure TCP keepalive on outbound HTTPS connections via a custom TCPKeepAliveHTTPAdapter
• Sets SO_KEEPALIVE, TCP_KEEPIDLE (300s), and TCP_KEEPINTVL (60s) to match server-side configuration
• Adds test verifying keepalive options are set on the adapter's pool manager

Based on #70 by @zaf, rebased onto current main. Closes #70.

Test plan

• All 37 existing tests pass
• New test_keepalive_socket_options_are_configured_on_adapter verifies options set correctly
• Verify keepalive behavior in production environment

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Note

Medium Risk
Changes the HTTP transport adapter used for all HTTPS requests, which can affect connection reuse and behavior across platforms (different socket options availability). Impact is limited to client networking settings and covered by a targeted unit test.

Overview
Enables TCP keepalive on all outbound HTTPS requests by replacing the default HTTPAdapter with a custom TCPKeepAliveHTTPAdapter that injects keepalive-related socket_options into both direct and proxy connection pools.

Adds configurable constants for keepalive timing (idle 300s, interval 60s) and a new test (test_keepalive_socket_options_are_configured_on_adapter) asserting the adapter preserves urllib3 defaults while adding the keepalive options when supported by the OS.

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