overload: add shutdown overload action

[salman-frs]

Fixes #41492.

An overload condition is meant to be transient, but some never clear. A memory leak or heap
fragmentation can hold the fixed heap monitor above the stop_accepting_requests threshold
indefinitely, so Envoy stays up rejecting everything, and no OOM killer fires because the process is
not actually at a hard limit. The only way out today is for something outside Envoy to notice and
restart it.

This adds a well known overload action, envoy.overload_actions.shutdown, with a required
ShutdownConfig. When the action stays saturated without interruption for saturation_duration,
Envoy drains and exits. Envoy does not restart itself, that is the
supervisor's job, hence shutdown rather than restart.

Requiring continuous saturation is what bounds the restart rate: any dip cancels the countdown, so a
freshly started Envoy has to be saturated for saturation_duration all over again before it can
shut down. max_jitter keeps a fleet that shares one overload condition from exiting in lockstep.

Shutdown reuses the existing graceful path rather than adding a second notion of drain time: fail
the health check, stop accepting, drain over --drain-time-s, exit on drain completion.

The countdown lives in source/server/overload_shutdown.cc rather than in OverloadManagerImpl
because it needs Server::Instance to drain and exit. It follows Memory::HeapShrinker.

Risk Level: low -- opt-in, nothing changes unless the action is configured.
Testing: unit tests in overload_shutdown_test.cc, config parsing tests in overload_manager_impl_test.cc, integration test in overload_integration_test.cc.
Docs Changes: new Shutdown section in docs/root/configuration/operations/overload_manager/overload_manager.rst, a row in the overload actions table, and an example config.
Release Notes: changelogs/current/new_features/overload_manager__shutdown-action.rst
Platform Specific Features: n/a.

[repokitteh-read-only]

Hi @salman-frs, welcome and thank you for your contribution.

We will try to review your Pull Request as quickly as possible.

In the meantime, please take a look at the contribution guidelines if you have not done so already.

🐱

Caused by: #46932 was opened by salman-frs.

see: more, trace.

[repokitteh-read-only]

CC @envoyproxy/api-shepherds: Your approval is needed for changes made to (api/envoy/|docs/root/api-docs/).
envoyproxy/api-shepherds assignee is @adisuissa
CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

🐱

Caused by: #46932 was opened by salman-frs.

see: more, trace.