Add watch support to namespace registry

[jprieto-temporal]

What changed?

Add watch support to the namespace registry. When persistence supports namespace watches, the registry receives real-time updates via a channel instead of periodic polling. On startup, the registry attempts to establish a watch. If successful, it processes create/update/delete events as they arrive. If watches aren't supported (ErrWatchNotSupported), it falls back to the existing polling behavior.

Other changes:

• Add WatchNamespaces method to the Persistence interface used by the registry
• Change stateChangeCallbacks from map to sync.Map to simplify callback handling from watches
• Add various namespace registry metrics

Why?

Watch-based updates provide lower latency for namespace changes compared to polling, and reduce load on the persistence layer.

How did you test it?

• built
• run locally and tested manually
• covered by existing tests
• added new unit test(s)
• added new functional test(s)

Potential risks

• Persistence implementations that don't support watches will continue using polling (no behavior change)
• Watch reconnection uses unlimited retries after initial startup succeeds, which could cause log spam if persistence becomes permanently unavailable