LibraryImport methods should be implicitly RequiresUnsafe

[jkoritzinsky]

Methods attributed with LibraryImportAttribute are implicitly unsafe. Although the marshalling logic could be analyzed to be considered memory-safe, the compiler cannot validate that the underlying extern DllImport is targeting a method with a valid signature.

Today, the stubs generated by LibraryImportGenerator will not generate the RequiresUnsafeAttribute on the produced stubs. This causes inconsistency with stubs that have no marshalling (they are just a raw DllImport, which is extern).

The LibraryImportGenerator should add the RequiresUnsafeAttribute to all generated stubs to provide a consistent experience.

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