Reinterop: Nullable, non-struct fields are not correctly accessed using std::optional

[kring]

From https://github.com/CesiumGS/cesium-unity/pull/211#discussion_r1115973471:

When I do a nullable non-struct field in a class (for example, I added public int? test2 to CesiumGeoreference, it's not generating code with std::optional, it's doing this instead:

public: ::DotNet::System::Nullable1<::std::int32_t> test2() const;
public: void test2(const ::DotNet::System::Nullable1<::std::int32_t>& value) const;

Is that intentional?