Updated Simple Event Sourcing with EventStoreDB sample

[oskardudycz]

1. Added Default object for ShoppingCart to get rid of nullability and forced not nulls with an exclamation mark,
2. Replaced EventStoreDBRepository with extension methods, as there is no expectation of having other implementation.
3. Added CommandHandlersBuilder to simplify the command handlers registration.
4. Removed underscores from stream category name to not be misleading with dashes.
5. Added Closed shopping cart status as bit flag of Confirmed and Cancelled to simplify the check in the business logic.
6. Removed redundant Shopping Cart status in the ShoppingCartInitialized event (it was always the same).

Thanks @bartelink for the feedback and suggestions!

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This discussion was created from the release Updated Simple Event Sourcing with EventStoreDB sample.