Update models across the SDK by removing optionals where needed

[nikola-mladenovic]

During the first iteration, we modeled everything as optional, mostly because the ObjectMapper which we're using for mapping from / to JSON responses / dictionaries, didn't support it properly at the time.

The goal here is to use ImmutableMappable protocol for those, read-only responses, and continue to use Mappable for others, while removing the optional property type where appropriate.