Add example for enabling TCP keepalive via socket_options

[thev1ndu]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

Configuration.socket_options is already passed through to the underlying urllib3 pool, but there is no example showing how to use it. Long-lived requests such as watches or follow=True log streams can hang when a connection is silently dropped by a load balancer or firewall. Enabling TCP keepalive lets the kernel detect the dead peer and close the socket. This adds examples/tcp_keepalive.py and lists it in the examples README. The helper only sets the platform-specific options (TCP_KEEPIDLE and friends) when they are available.

Which issue(s) this PR fixes:

Fixes #2067

Special notes for your reviewer:

Example only, no library code changes.

Does this PR introduce a user-facing change?

NONE

[kubernetes-prow]

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• 1a3b51b Add example for enabling TCP keepalive via socket_options

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[aojea]

Long-lived requests such as watches or follow=True log streams can hang when a connection is silently dropped by a load balancer or firewall

TCP_KEEPALIVE is not the right solution, it does not work as people thing it does, once the connection is established the keepalives are not sent periodically ... http2 ping frames is the solution to idle connections and middleware silent drops kubernetes/kubernetes#95981