Debug libs with PDBs or symbol server

From https://github.com/CesiumGS/cesium-unreal/pull/235#discussion_r596492068

> It could be useful in the future to have a fully-debuggable plugin package pop out of CI. To do that, we would need to either:
>
> 1. include PDBs in the package, or
> 2. make them available separately from a symbol server.
>
> We should also index our PDBs with source information. I haven't tried it, but these instructions sound right to me: gist.github.com/baldurk/c6feb31b0305125c6d1a
>
> But I think that's outside the scope of this PR, and the first release of Cesium for Unreal.