In each database, persist the right Rule and not the whole schema

[danovaro]

Is your feature request related to a problem? Please describe.

every time we visit a new database, we have to parse the schema and find the matching rule. Now that we have Schema/Rule serialisation, we should persist (and stream, in the case of RemoteFDB) only the correct rule

Describe the solution you'd like

remove the Schema ptr in the Rules
add the ability to extract the Rule, whenever the DB only contains the Schema

Keep the original schema for backward compatibility (at least in a transition phase)

Describe alternatives you've considered

No response

Additional context

No response

Organisation

ECMWF