Add era5 virtualizarr example

[mpiannucci]

Just a single file for now, starting point for building large virtual version

[TomNicholas]

In general I would encourage using vds vs ds as a convention to distinguish between datasets that contain ManifestArrays and those that don't. It's confusing otherwise because the top-level type is fundamentally unhelpful (see zarr-developers/VirtualiZarr#171).