[dotnet-svcutil] defer known types import to resolve IsReference inheritance issue

[imcarolwang]

Fixes #5769: The DataContract IsReference property was incorrectly set to false for some derived types when known types and inheritance were involved. This led to a mismatch between the imported DataContract type and the referenced type, which caused issues when trying to reuse the referenced types.

Root cause: ImportKnownTypes(typeName) was called immediately after importing each type, which could import derived types as known types before their base types were fully processed, causing inheritance-based IsReference propagation to fail.

Solution: Implement deferred known types import strategy:

• Add _deferredKnownTypesImport list to collect types requiring known types import
• Defer known types import until after all main types are imported
• Process deferred known types after inheritance chains are fully established
• Ensure IsReference property propagates correctly through inheritance hierarchy

This ensures that inheritance-based property propagation works correctly by processing known types only after the complete type hierarchy is established.