Backfill newly injected attributes

[jbusecke]

In the future we might refactor our tests to depend on attributes injected by the recipe ( like here).
These attributes will only be available on very recent stores.

We have the following options:

• Strictly dismiss anything that does not contain the current set of required metadata (that would be too harsh)
• Once we settle on a set of metadata to include we should see if it is possible to have a 'backfill script' that iterates through all stores, and uses existing metadata to fill dataset_level meta data by querying the ESGF API -- I don't think we will be able to deduce file level data since those old stores do not retain enough info -- and overwriting the zarr stores. (they should clearly be marked by some additional attribute like pangeo_forge_metadata_backfilled or similar.