Add Redis Cluster support

[fatihergin]

Addresses https://github.com/hexcowboy/rslock/issues/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