Fix generated asset/assetId property for ffi-native

[nmfisher]

Using asset under ffi-native generates an incorrect asset property rather than assetId (reference), which fails to compile. This PR:

• changes the generated annotation property from asset to assetId
• changes the YAML key from asset to assetId (technically a breaking change to existing configs, but these were presumably broken anyway)
• adds tests for ffi-native

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[dcharkes]

Thanks @nmfisher! ❤️

technically a breaking change to existing configs, but these were presumably broken anyway

We're bumping the version to 10.0 anyway, so that should be fine.

It might have been that people were using the deprecated @FfiNatives, but I doubt it due to the native assets feature not having landed yet.

[dcharkes]

Thanks @nmfisher! 🎸