chore: expose query_redis_for_key_value

[xangelix]

Currently, LockManager abstracts away the underlying Redis connections, which prevents users from inspecting the value of a lock they failed to acquire without creating a separate, redundant Redis client.

In distributed systems where a lock represents "Leader" or "Worker" ownership of a resource, the lock's value often contains the address (e.g., DNS name or IP) of the current owner.

If a client fails to acquire a lock (lock() returns Err), they often need to "follow" the lock by reading its value to find out who currently owns it (e.g., to proxy a request to the active leader).

This allows users to reuse the existing connection pool managed by rslock to perform these lookups, rather than managing a secondary Redis pool just for inspection.

// Example usage enabled by this PR:
let manager = LockManager::new(uris);

match manager.lock(resource, ttl).await {
    Ok(lock) => { /* I am the leader */ },
    Err(_) => {
        // I am not the leader, but I can now find out who is:
        if let Ok(Some(value)) = manager.query_redis_for_key_value(resource).await {
             let leader_address = String::from_utf8(value).unwrap();
             // proxy_request(leader_address, ...);
        }
    }
}