Implement earthkit xarray engine

[sandorkertesz]

The task is to continue the developments in branch feature/earthkit-engine to develop a new xarray engine to replace cfgrib for grib -> xarray -> grib conversions.

[sandorkertesz]

Still work in progress. Notebooks showing how the new engine works can be found at:

https://earthkit-data.readthedocs.io/en/feature-earthkit-engine/examples/index.html#xarray-engine

and

https://github.com/ecmwf/earthkit-data/tree/feature/earthkit-engine/docs/experimental

The documentation of the options is available here (see the open_dataset method):

https://earthkit-data.readthedocs.io/en/feature-earthkit-engine/_api/data/utils/xarray/engine/index.html#data.utils.xarray.engine.EarthkitBackendEntrypoint