Wrapped vs non-wrapped variants of `AbstractDataType` for codegen model

[desaikd]

Currently AbstractDataType has 2 variants for sequence/scalar types(i.e. wrapped vs non-wrapped). The non-wrapped variant here is always generated as nested part of some other class and might not require to be represented as a model because the details for it can be stored in the class itself. Currently, its FullyQualifiedTypeReference name is getting stored in the TypeStore and the fields property of a structure(class) has a reference to it. Look into removing these non-wrapped variants and see if it has any requirements to keep it for templates.

More discussion: https://github.com/amazon-ion/ion-cli/pull/144#discussion_r1765910844