Modify `AbstractDataType` to be independent from target programming language

[desaikd]

Currently ADT has a dependency on the target programming language through namespace paths. The FullyQualifiedTypeName and FullyQualifiedTypeReferences in ADT use target programming language to determine the namespace path format. (For example, Java uses class/enum names but Rust uses a module name + struct/enum name)

The ADT should just store the namespace as strings and on the template side or generation side it should properly format namespace path based on the target programming language.

Reference discussion: https://github.com/amazon-ion/ion-cli/pull/163#discussion_r1823154978