tlsutil: add CAReloader for dynamic CA certificate reloading

[aaomidi]

Summary

Add CAReloader that polls CA files and updates the certificate pool when changes are detected (using SHA256 hashing). This enables zero-downtime CA rotation for mTLS clusters.

Changes

• Add CAReloader struct with Start()/Stop() lifecycle management
• Use sync.RWMutex for thread-safe pool access
• Use sync.WaitGroup for graceful shutdown (follows etcd patterns)
• Graceful fallback: invalid CA files keep using cached pool
• Add Prometheus metrics for reload operations (etcd_tlsutil_ca_reload_*)

Background

This PR is a split of #21074 into smaller, reviewable pieces as requested by maintainers.

Stacking

This PR is part of a stacked PR series for CA hot-reload support (issue #11555):

1. This PR - tlsutil: add CAReloader
2. transport,embed: integrate CA hot-reload support #21157 - transport,embed: integrate CA hot-reload support
3. tests: add CA rotation integration test and manual testing tools #21158 - tests: add CA rotation integration test

---

Part 1 of 3 for CA hot-reload support.
Replaces #21074 (split into smaller PRs)
Partially fixes #11555

Signed-off-by: Amir Omidi amir@aaomidi.com

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @aaomidi. Thanks for your PR.

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