This PR makes the marker interface `INotification` optional.

[vvdb-architecture]

INotification is an unnecessary constraint. It's absence does not prevent the correct functioning of INotificationHandler/INotificationPublisher.

This avoids the creation of a specific type for each notification type, and allows us to define handlers using value tuples:

public class Entity
{
}

public enum Operation
{
  Create,
  Update
}

public class EntityEventHandler : INotificationHandler<(Entity Entity, Operation Operation)>
{

    public Task Handle((Entity Entity, Operation Operation) notification, CancellationToken cancellationToken)
    {
    }
}

... and then call them using a syntax like:

        mediator.Publish((entity, Operation.Create));

[github-actions[bot]]

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[jbogard]

The INotification interface is also there to prevent mistakes. Not publishing requests or sending notifications. I don't want to go back on that just to enable tuples.