Add cupy support

I'm intending on working on cupy support in xarray along with @quasiben. Thanks for the warm welcome in the xarray dev meeting yesterday!

I'd like to use this issue to track cupy support and discuss certain design decisions. I appreciate there are also issues such as #4208, #3484 and #3232 which are related to cupy support, but maybe this could represent an umbrella issue for cupy specifically.

The main goal here is to improve support for array types other than numpy and dask in general. However, it is likely there will need to be some cupy specific compatibility code in xarray. (@andersy005 raised issues with calling __array__ on cupy in #3232 for example).

I would love to hear from folks wanting to use cupy with xarray to help build up some use cases for us to develop against. We have some ideas but more are welcome.

My first steps here will be to add some tests which use cupy. These will skip in the main CI but we will also look at running xarray tests on some GPU CI too as we develop. A few limited experiments that I've run seem to work, so I'll start with tests which reproduce those.

Start with related issues #4208, #3484, and #3232 and the initial CuPy experiments described here. Add CuPy-focused tests that reproduce the working cases, then use their results to define compatibility needs and assess whether GPU CI is needed.