xsd-model: create abstract code level structs for all schema-top types

In the branch xsd-model, we need to create abstract code level structs for all types from xsd-parser/src/xsd_model/groups/schema_top.rs. In the scope of the same task, we need to implement conversions from XSD model level types to abstract code level types.

List of types:

[ ] TopLevelSimpleType
[ ] TopLevelComplexType
[ ] Group
[ ] AttributeGroup
[ ] TopLevelElement
[ ] TopLevelAttribute
[x] Notation

lumeohq / xsd-parser-rs

xsd-model: create abstract code level structs for all schema-top types #98