Work out how to download ERA5 data on the IceNet grid

[jemrobinson]

IceNet has a dataloader for ERA5 that downloads data onto the 432x432 grid. It would be nice if we could get anemoi-datasets to do this for us (instead of one of their default grids). I've opened an issue here ecmwf/anemoi-datasets#373 to ask if it's possible.