Add Unreal Engine 5.8 compatibility

[nkmlombardi]

What changed

• add the Blueprint categories now required by UnrealHeaderTool for exposed properties;
• pass shared JSON member names in the form required by Unreal Engine 5.8;
• add explicit headers that are no longer supplied transitively;
• clean up the malformed SESSION_ID_KEY macro declaration.

Why

Unreal Engine 5.8 tightened UHT validation and changed shared JSON key handling. The current plugin consequently fails to compile when used from a UE 5.8 source build.

These source-level migrations preserve the existing GSDK behavior while allowing UE 5.8 projects and dedicated servers to build the plugin.

Validation

• packaged the plugin for UE 5.8 Editor, Game, and Shipping targets;
• built a real UE 5.8 UE_SERVER dedicated-server target with the plugin;
• cooked, staged, and launched that dedicated server;
• verified local -NoGSDK initialization and ready-for-players lifecycle with no fatal error or assertion.

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