C++ SDK gets typed metadata

C++ client now ships the run + unit metadata feature with a typed std::variant for the value instead of a bare nlohmann::json.

• Regenerated C++ SDK against the new metadata endpoints
• Metadata field is now std::map<std::string, std::variant<std::string, double, bool>> so values keep their type at compile time
• Added Custom Metadata sections to the runs and units guides with snippets for Python, C#, C++, and MATLAB