Seamless era5 connection through Zarr

Saw on [LinkedIn](https://www.linkedin.com/posts/%F0%9D%97%98%F0%9D%97%A5%F0%9D%97%94%F0%9D%9F%B1-%F0%9D%97%B6%F0%9D%97%BB-%F0%9D%98%81%F0%9D%97%B5%F0%9D%97%B2-%F0%9D%97%B0%F0%9D%97%B9%F0%9D%97%BC%F0%9D%98%82%F0%9D%97%B1-%F0%9D%97%BB%F0%9D%97%AE%F0%9D%98%81-share-7478464368662233088-3WYy/?utm_source=social_share_send&utm_medium=android_app&rcm=ACoAACDh3_8BRsxrOI8mW3Ys43ylJt_smAbIf_I&utm_campaign=copy_link) that era5 is now accessible as Zarr, making forcing much simpler possibly? https://github.com/ecmwf-training/dss-notebooks/blob/main/datasets%2Freanalysis-era5-single-levels%2Farco-access.ipynb.

Start with the linked arCO-access.ipynb notebook to understand how ERA5 is exposed through Zarr. Then inspect this project's existing forcing and data-access entry points; done should mean ERA5 Zarr data can be connected through the package with a clearly documented, working path.

Written by the indexing model from the issue text.