Reuse top-level prefix for all extension base types with explicit XML type declarations

[glopesdev]

WorkflowBuilder serialization procedure has been modified to walk up the inheritance tree of each extension type to search for any base types with explicit XmlTypeAttribute declarations. If any exist, they are explicitly added to the set of serializable extension types.

This prevents overly verbose inline XML namespace declarations for properties provided by base types which are not directly used elsewhere in the workflow.

Fixes #1056

[glopesdev]

The initial version of this fix was aiming at resolving ambiguous XmlSerializer behavior when serializing properties from base types without explicit XmlTypeAttribute declaration.

This approach has been dropped in favor of unambiguous serialization for types with explicit attribute declarations. In general, base types with serializable properties should always explicitly declare XmlTypeAttribute to ensure predictable namespace resolution.