feat(valkey): add Valkey addon with sentinel-based replication

[weicao]

Summary

Add a new Valkey addon supporting sentinel-based replication topology (3 data pods + 3 sentinel pods).

Features

• Topology: Sentinel-based replication (primary + replicas, 3 sentinels for HA)
• Versions: Valkey 8.x and 9.x
• Operations: All standard KubeBlocks OpsRequests (VScale, HScale, Restart, Stop/Start, Upgrade, Reconfiguring, Expose, Backup/Restore, TLS, ACL, Switchover, RebuildInstance)
• Backup: Physical backup via kopia with pre/post-restore sentinel re-registration
• TLS: Full TLS support including sentinel TLS replication

Key design decisions

• Sentinel uses emptyDir (no PVC) — conf is rebuilt from scratch on every restart via _background_monitor_discovery background loop
• valkey-start.sh uses quorum-based sentinel query (majority of sentinels must agree on the same master) before falling back to lexicographic heuristic — prevents split-brain during sentinel convergence windows
• valkey-member-leave.sh selects the highest config-epoch sentinel (avoids isolated/stale sentinels); skips SENTINEL RESET unless a SENTINEL FAILOVER was actually triggered and completed — premature RESET temporarily zeros num-slaves and can cause restarting pods to elect a second master
• switchover.sh uses strict wait_for_new_master || return 1 for targeted switchover to ensure KubeBlocks reports failure if the wrong candidate was elected