Add Field Casting Support for Lexicon Types

[edevsh]

• Add contracts for FieldType handler and factory architecture
• Refactor LexiconContract::nsid to be static
• Integrate FieldType with BaseResponse casting
• Fix broken GetTimelineTest
• Add castable field classes for array, object, and union types
• Implement abstract and concrete definitions for array, object, and union field types
• Add type pair factories for array, object, and union fields
• Add handlers for array, object, and union field types
• Add FieldType API for creating typed field definitions
• Add unit tests for field type definitions, handlers, factories, and castables
• Add feature test for full field type casting flow

fixes #61