Unable to override Cesium credit system because `CesiumCredit` is marked internal

For people that want to change how Cesium for Unity renders credits, we recommend the steps here:
https://github.com/CesiumGS/cesium-unity/issues/333#issuecomment-1583488794

But this doesn't seem to work anymore because `CesiumCredit` is internal, so it's impossible to follow those instructions from outside the CesiumRuntime assembly. But the change from public to internal happened _before_ we wrote those instructions, and they did seem to work when they were written, so it's not clear what's happening. We should investigate. Most likely the solution will simply be to make `CesiumCredit` and related types public.

As reported here:
https://github.com/CesiumGS/cesium-unity/issues/333#issuecomment-2002638298