Add a way to extend JSON-LD models

[RomanovM]

I don't see how use the JsonLdCreatedEvent would work. Its "parameter" is still the JsonLdProductModel, not the JsonLd string property that gets "emitted" to the web page.

(I think the JsonLdProductModel needs some custom property like (?)
public string Dictionary<string,object> CustomProperties { get; set; }

Source: https://www.nopcommerce.com/boards/topic/101722/new-microdata-in-470-configuration