CNPG should validate memory resources settings against the shared_memory Postgres parameter #2664

In #2661, we added a validator to catch some possible mistakes in resource definition.

We can expand it to catch more obvious mistakes in the memory settings, comparing them to the shared_memory Postgres configuration parameter.

If defined, the memory resources must be greater than the Postgres shared memory.

We could also talk about it if we want to let the user know (with a warning) if the values are too narrow, but that would be a heuristic and may require another ticket to introduce it.