Verify unmanaged callbacks

[badcel]

Unmanaged callbacks can be called directly from c without needed to allocate a delegate. This improves performance as no marshaling is needed and could simplify the generated code as there would be no delegate which must be kept alive.

https://learn.microsoft.com/en-us/dotnet/api/system.runtime.interopservices.unmanagedcallersonlyattribute?view=net-7.0