Parametrize and unify base endpoints for instance, cluster and jobs

[Joscorbe]

I think we can unify these 3 endpoints in a single one, similarly as the metadata or attributes method.
These 3 endpoints have exactly the same code inside, they just call a PostgREST RPC which can be parametrized depending on the called URL.

[icot]

This may not make sense when we consolidate the schemas and start taking steps to remove postgrest from the picture, so no need to do it now. We'll revisit when the dbod/apiato merge has been completed