Add Redis Cluster support

[fatihergin]

Addresses #21.

Please review the Docker setup (tests) with extra care, as I'm new to Docker.

Changes

• Add Redis Cluster support via:
  • LockManager::new_cluster(uris: Vec<Vec<T>>)
  • LockManager::from_cluster_clients(clients: Vec<ClusterClient>)
• Add cluster integration tests (single_cluster, multi_cluster)
• Add Docker Compose test environment with 3 Redis clusters
• Update README with cluster usage and locking behavior

Testing

Test single-cluster (simple lock) and multi-cluster (Redlock) behavior:

# Executes both single and multi-cluster examples
docker compose -f examples/docker-compose-cluster.yml up --build

[hexcowboy]

Thanks @fatihergin for putting this together. We decided to proceed with a separate implementation which keeps cluster support opt-in and accepts a preconfigured ClusterClient for advanced configuration. We also scoped each manager to one logical Redis Cluster to avoid conflating cluster seed nodes with independent Redlock quorum members. We really appreciate your contribution -- it helped clarify the design tradeoffs.