Support for global models

[frazane]

This PR introduces minimal support for global models in evalml, as we might want to evaluate small experiments on coarse global grids (e.g. o48 or o96).

Summary of changes

• optionally disable setting the ECCODES_DEFINITION_PATH environment variable when running global models
• added base anemoi-inference config for global models and an example evalml config
• switch from meteodatalab to earthkit-data (with its xarray engine) to load GRIB files during verification