Non-sealed classes implementing 'IAsyncDisposable' should provide a 'DisposeAsyncCore' method

Non-sealed classes implementing `IAsyncDisposable` should provide a `virtual` and `protected` parameter-less `DisposeAsyncCore` method that returns a `ValueTask`. The signature should be:

```c#
protected virtual ValueTask DisposeAsyncCore()
{
}
```

(see https://learn.microsoft.com/en-us/dotnet/standard/garbage-collection/implementing-disposeasync )