feat(compiler): add union support to fory compiler and runtime

[chaokunyang]

Why?

Fory needs Union type support in its IDL compiler to enable cross-language serialization of sum types (tagged unions). This allows users to define Union types in FDL/Protobuf/FlatBuffers schemas and have the compiler generate code for all supported languages (Java, C++, Python, Go, Rust).

What does this PR do?

This PR adds comprehensive Union type support across the entire Fory stack:

Compiler & IDL:

• Added Union AST node to the IR with support for FDL, Protobuf (oneof), and FlatBuffers (union)
• Implemented Union code generation for all languages: Java, C++, Python, Go, and Rust
• Added validation and semantic checks for Union types
• Updated parsers to recognize Union/oneof/union syntax in FDL, Proto, and FBS formats

Runtime Support:

• Java: Implemented generic UnionSerializer for generated Union classes, supporting Union2-Union6 typed variants
• C++: Added union_serializer.h with template-based serialization for unions, integrating with existing std::variant support
• Python: Created union.py module with UnionSerializer for dynamic union type handling
• Go: Implemented union.go with union type registration and serialization
• Rust: Enhanced enum derive macro to generate Union-compatible serialization with proper TypeId normalization

Protocol & Xlang:

• Aligned cross-language Union serialization format: index + ref_flag + type_id + data
• Updated xlang specification to document Union type behavior
• Added named union support with meta-sharing for efficient type encoding
• Implemented skip logic for Union types in all languages

Testing:

• Added integration tests in idl_tests for C++, Go, Java, Python roundtrip validation
• Added xlang tests to verify cross-language Union compatibility

Related issues

Closes #3182 #3191 #3170
#1017
#3099

Does this PR introduce any user-facing change?

Yes. Users can now define Union types in Fory IDL files and use Protobuf oneof or FlatBuffers union syntax which will be compiled to Fory Union types.

• Does this PR introduce any public API change? Yes - Adds Union types to the type system
• Does this PR introduce any binary protocol compatibility change? No - Union uses existing TypeId (31) from xlang spec

Benchmark

No performance regression expected. Union serialization uses the same optimized paths as existing reference serialization with additional index metadata.