ext_proc: respond with 504 to downstream on timeout

[cainelli]

trafficstars

Commit Message: ext_proc: respond with 504 to downstream on timeout Additional Description: Return 504 Gateway Timeout to downstream on timeout errors. Risk Level: Low Testing: Local build and a slow extproc implementation.

Fixes https://github.com/envoyproxy/envoy/issues/34843

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:cat:

Caused by: https://github.com/envoyproxy/envoy/pull/34856 was opened by cainelli.

see: more, trace.

[tyxia]

/wait

[cainelli]

Not sure why it fails checks/start.

/retest