Add data contract conditional compilation variable

Related to this simple-json PR:
https://github.com/facebook-csharp-sdk/simple-json/pull/26

Per @prabirshrestha's [comment](https://github.com/Haacked/simple-json/commit/33c53db075451703f9e52019db4e85884e718290#commitcomment-2404336) I made my changes in Reflection Utils first.

However, it's a bit weird because I'm defining a conditional constant that's specific to simple json here, `REFLECTION_UTILS_DATA_CONTRACT` and sort of doesn't make sense.

But if we don't define this constant here, it makes it harder to "copy and paste" this code to simple-json. Let me know what you think.