Add Unreal Engine 5.8 generated SDK compatibility

[nkmlombardi]

What changed

• update the JavaScript generator and EJS templates for Unreal Engine 5.8 JSON key handling;
• migrate deprecated shrinking flags to EAllowShrinking;
• emit PlatformAllowList in the plugin descriptor;
• regenerate the checked-in UE 5.7 plugin output from those templates.

Why

The current generated Marketplace plugin fails to compile under Unreal Engine 5.8 because shared JSON member names no longer match the accepted FJsonObject key type, older array shrinking booleans are no longer accepted at the affected call sites, and the descriptor uses legacy platform-list terminology.

Updating the generator is important here: changing only the checked-in generated C++ would cause a later SDK generation pass to reintroduce the incompatibilities.

Impact

Generated API payload field names and runtime behavior are unchanged. The patch updates the emitted C++ and descriptor syntax needed to consume the SDK from UE 5.8.

Validation

• regenerated the UE 5.7 plugin output with the patched templates;
• packaged the generated plugin against an Unreal Engine 5.8 source build for Editor, Development, and Shipping configurations;
• built it as part of a UE 5.8 game Editor and Client integration.

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