Move support for CUDA to extension

[Sbozzolo]

It would be nice for ClimaCore not to depend directly on CUDA.

Right now, packages that depend on ClimaCore have to download and instantiate CUDA even if they are not able to use it (e.g., on Github Action runners). The CUDA runtime is not exactly tiny, which adds to the initialization costs.

Moving to an extension would also set a precedent for supporting different backends.

(In many ways, this seems to be precisely the use case extensions were designed for.)