async support for ITransactionScope

[Andriy1024]

Dear developers,
I faced the challenge to trigger some async work on the transaction completed event.
The current ITransactionScope's Complete method is not async compatible. It would be nice if you consider adding async support for this case.
Thank you!

The first option is to make 'Complete' methods async. In this case, it would be also nice to make it virtual in 'DefaultTransactionScope'.

public interface ITransactionScope : IDisposable
{
    /// <summary>
    /// Completes a transaction (commits or discards the changes).
    /// </summary>
    ValueTask CompleteAsync();
}

The second option is to use IAsyncDisposable

public interface ITransactionScope : IAsyncDisposable
{
...
}