This PR makes the exported DllGetActivationFactory zero-alloc, skipping the useless string runtimeClassId parameter.
It does so by introducing the following new API to MarshalString:

public static ReadOnlySpan<char> FromAbiUnsafe(IntPtr value);

This allows marshalling an HSTRING to a ReadOnlySpan<char> value, without allocating a temporary string. The "Unsafe" suffix matches the naming convention in the BCL for methods that are not safe when misused. In this case, callers have to make sure not to escape the returned ReadOnlySpan<char> value outside of the scope where the HSTRING is valid.

Then, it updates the generated Module type so that when not on .NET Standard, the GetActivationFactory method will actually take a ReadOnlySpan<char> parameter (with a second string overload just forwarding to it). The new native exports for NativeAOT can then just call this new ReadOnlySpan<char> overload using the new FromAbiUnsafe marshalling method.