feat(go): use option for optional fields

[chaokunyang]

Why?

Align Go codegen/serialization with xlang optional field semantics and add cross-language coverage for optional IDL types.

What does this PR do?

• Generate Go optional fields using optional.Optional[T] (including decoder/encoder codegen, struct metadata, and type resolution) instead of pointers where supported.
• Add optional-types IDL schema and cross-language roundtrip tests for Java, Go, Python, Rust, and C++ integration tests.
• Update C++ timestamp serialization to microsecond precision per spec and expand field-config macro support.
• Adjust Python xlang struct fingerprinting for user-defined types and add xlang byte-array read/write helpers.
• Ignore generated optional-types artifacts in integration test fixtures.
• Make struct object always addressable by copy unaddressable struct into a temp object to remove reflect from whole codepath

Related issues

Closes #3196

Does this PR introduce any user-facing change?

• Does this PR introduce any public API change?
• Does this PR introduce any binary protocol compatibility change?

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