Add utility methods to scaffold e.g. single block server side

[bjarnef]

When creating/updating content nodes service side it may fail, if the content type is using single block property (e.g. for a top banner/image) even though one don't care about this property when importing data.
#17817

From backoffice one won't notice as this is scaffolded client side via blockEditorService.createModelObject().
So it "magically" creates the required block data.

Same may happen if Block List / Grid requires a minimum amount of blocks.

Mainly when dealing with single blocks I think this can be simpler.
We can of course construct the JSON ourselves:
https://docs.umbraco.com/umbraco-cms/13.latest/fundamentals/backoffice/property-editors/built-in-umbraco-property-editors/block-editor/block-grid-editor#creating-a-block-grid-programmatically

I probably think it is kinda missing an equivalent method service side for CreateModelObject() .. probably based on content type or optionally passing in ModelsBuilder model CreateModelObject<T>().