[RFP] Replace LI's object with interface in delivery models

[sevcik-martin]

Proposed solution

Replace LI's IEnumerable<object> with IEnumerable<IContentItem> for delivery models. Causing a breaking change for delivery models.

Additional context

Be aware that this will intorduce a breaking change.
IContentItem will be created in #90.
Blocked by #90.

[sevcik-martin]

Interface IContentItem was created as part of Delivery.Abstractions kontent-ai/delivery-sdk-net#368.
Support for generating models implementing IContentItem interface and replacing object of modular content elements with IContentItem will be done as part of #90 using the structuredModel option.