Add support for Arrow Union and Avro Union resolution and improvements in Avro union validation

- Expanded `arrow-avro` to support resolving Arrow unions to Avro unions including validation against duplicate branch types, nullability handling, and prohibition of nested unions.
- Refactored `Codec`, schema resolution, and added new utility functions for union branch management.
- Introduced dense union field mapping, Arrow metadata persistence, and Avro union branch validation.
- Extended tests to cover union scenarios including dense mode, metadata persistence, and type promotion.

Author: jecsand838