tlsutil,transport,embed: add CA trust bundle hot-reload support

[aaomidi]

Fixes #11555

Add support for dynamically reloading trusted CA certificates without requiring server restart. This enables zero-downtime CA rotation for mTLS clusters.

Key changes:

• Add CAReloader in tlsutil that polls CA files and updates the certificate pool when changes are detected (using SHA256 hashing)
• Integrate CA reload into transport layer for both server and client TLS
• Add --peer-tls-reload-ca, --client-tls-reload-ca, and --tls-ca-reload-interval flags to etcd server
• Add Prometheus metrics for CA reload operations
• Add integration test for CA rotation scenarios

The feature is disabled by default and must be explicitly enabled via the new flags.

I am happy to split this up as most of this PR is just the various integration/e2e tests.

[k8s-ci-robot]

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