PIP-121: Implement AutoClusterFailover

[BewareMyPower]

Compared to the Java implementation, this PR changes the delay-based failover and switch-back configs to counter-based configs. See apache/pulsar#25326 for the reason.

[Copilot]

Pull request overview

Adds an AutoClusterFailover implementation of ServiceInfoProvider to automatically switch the client’s service URL between a primary cluster and fallback clusters based on periodic TCP reachability probes, plus unit tests for basic failover/switchback behavior.

Changes:

• Introduces new public pulsar::AutoClusterFailover API (builder/config) and implementation with periodic probing and delayed failover/switchback.
• Adds new tests exercising failover to an available secondary and switching back to primary after recovery.
• Makes ServiceInfo default-constructible to support the new builder/config construction pattern.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 7 comments.

File	Description
lib/AutoClusterFailover.cc	Implements the async probing + state machine for primary/secondary switching.
include/pulsar/AutoClusterFailover.h	Adds the public API (Config + Builder) for wiring AutoClusterFailover into Client::create(...).
include/pulsar/ServiceInfo.h	Adds a default constructor to ServiceInfo to enable default construction in the new builder/config.
tests/ServiceInfoProviderTest.cc	Adds probe-based tests for failover and switchback timing behavior.

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[Copilot]

Pull request overview

Introduces a new AutoClusterFailover ServiceInfoProvider implementation that periodically probes configured Pulsar service URLs and automatically switches between a primary and one (or more) secondary clusters based on availability.

Changes:

• Added public pulsar::AutoClusterFailover API (builder/config) in include/pulsar/AutoClusterFailover.h.
• Implemented probing + failover/switchback logic using Asio in lib/AutoClusterFailover.cc.
• Added new gtest coverage for failover and switchback timing behavior in tests/ServiceInfoProviderTest.cc.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 7 comments.

File	Description
include/pulsar/AutoClusterFailover.h	New public header/API for configuring and constructing the failover provider.
lib/AutoClusterFailover.cc	Core implementation: periodic async TCP probing and cluster switching logic.
tests/ServiceInfoProviderTest.cc	Adds tests exercising failover to secondary and switchback to primary after recovery.

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