Use Unreal's version of tinyxml2 in UE 5.5

[kring]

Starting in UE 5.5, Epic added tinyxml2 headers and libs to C:\Program Files\Epic Games\UE_5.5\Engine\Source\ThirdParty\TinyXML2. By using these in our builds (using a vcpkg overlay port), rather than bringing our own, we should be able to avoid conflicts with the NDisplay plugin.

https://community.cesium.com/t/cesium-tinyxml2-with-ndisplay-error-on-launch/38323

We'll have to continue to bring our own tinxml2 in older UE versions, though.