Add Redis Cluster support

[hexcowboy]

Summary

• add opt-in Redis Cluster support through a new cluster feature
• add LockManager::new_cluster for seed-node URIs and LockManager::from_cluster_client for advanced client configuration
• preserve the existing standalone Redis API and behavior
• add a six-node Redis Cluster lifecycle test covering lock, query, extend, and unlock
• add a cluster example and update the README for version 0.9.0

Why

The existing connection path only creates standalone multiplexed Redis connections, which cannot discover cluster topology or follow cluster routing. The new internal connection abstraction supports both standalone and topology-aware cluster connections while keeping cluster dependencies optional.

A supplied URI list represents seed nodes for one logical Redis Cluster, so it counts as one server for quorum purposes. Callers that need authentication, TLS, address remapping, or other advanced settings can supply a configured redis::cluster::ClusterClient.

Validation

• cargo test --all-features --all-targets (29 passed)
• cargo test --all-targets (25 passed)
• cargo clippy --all-features --all-targets -- -D warnings
• cargo fmt --all -- --check
• RUSTDOCFLAGS='-D warnings' cargo doc --no-deps --all-features

Closes #21