refactor: Implement FDv2 Wire Format + Payload Parser

[beekld]

This is a pretty straightforward port of FDv2ChangeSet and related classes from Java, as well as the protocol objects, such as PutObject, DeleteObject, etc.

• Adds FDv2Change, FDv2ChangeSet objects for use in data system.
• Adds ServerIntent, PutObject, etc methods for parsing messages from backend.
• Adds tag_invoke methods based on the existing json deserialization code.
• Adds tests for all of the JSON parsing.

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Note

Medium Risk
Introduces new protocol-level JSON deserialization and data model types that will be used for streaming/data-sync, so schema/compatibility mistakes could affect data updates despite being mostly additive and covered by unit tests.

Overview
Adds initial FDv2 wire-format support by introducing Selector, FDv2Change, and FDv2ChangeSet types for representing full/partial/no-op data updates.

Implements JSON deserialization (Boost.JSON tag_invoke) for FDv2 protocol messages—IntentCode, ServerIntent, PutObject, DeleteObject, PayloadTransferred, Goodbye, and FDv2Error—and wires the new parser into the internal build (serialization/json_fdv2_events.cpp).

Adds comprehensive unit tests (fdv2_serialization_test.cpp) validating successful parsing, null handling, and schema-failure behavior for missing/invalid fields.

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