Adjust cbor_types() for unknowable conditions

[rooooooooob]

e..g extern types, custom serialization, etc

not done right now as it impacts both #228 and #234

[rooooooooob]

Possibly break @custom_serialize into two categories: one which respects the CBOR type, and one that is 100% arbitrary.

It would be nice to be able to specify this for __CDDL_CODEGEN_EXTERN_TYPE__ too. Right now we have to provide dummy definitions in a separate dependency folder then delete them after instead of using __CDDL_CODEGEN_EXTERN_TYPE__ when the type is either a basic group OR it is a primitive. We still use it for fully-contained types (i.e. ones that contain their encoding information and are just regular structs).