Add health check support for Persistence journal and snapshot store

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Motivation

In cloud-native and Kubernetes deployments, it's critical to verify that persistence backends (journals and snapshot stores) are healthy, connected, and ready to serve reads/writes. Currently, pekko provides ClusterShardingHealthCheck for cluster sharding readiness, but there is no equivalent health check mechanism for the persistence subsystem.

Without persistence health checks, operators cannot:

• Verify journal connectivity during pod startup (readiness probes)
• Detect journal degradation or disconnection during runtime (liveness probes)
• Monitor recovery status of persistent actors

Proposed Enhancement

Add health check support to the persistence subsystem, following the pattern established by ClusterShardingHealthCheck:

1. A health check extension for AsyncWriteJournal that verifies the journal is reachable and responsive
2. A health check for snapshot store connectivity
3. Integration with Kubernetes-style readiness/liveness probe mechanisms

The akka.net project has implemented this feature and found it valuable for production deployments.

References

This enhancement was inspired by the Akka.NET project: akkadotnet/akka.net#7840