Add distributed lock support

[imperugo]

Summary

Add distributed locking via SE.Redis's built-in `LockTake`/`LockRelease`/`LockExtend`.

Proposed API

```csharp
Task LockTakeAsync(string key, string value, TimeSpan expiry, CommandFlags flag = CommandFlags.None);
Task LockReleaseAsync(string key, string value, CommandFlags flag = CommandFlags.None);
Task LockExtendAsync(string key, string value, TimeSpan expiry, CommandFlags flag = CommandFlags.None);
Task<string?> LockQueryAsync(string key, CommandFlags flag = CommandFlags.None);
```

Considerations

• Document the lock value pattern (use unique identifier per holder)
• Consider an `IAsyncDisposable` wrapper: `using var lock = await redis.AcquireLockAsync("key", TimeSpan.FromSeconds(30))`
• Consider retry logic with configurable backoff