persistent only subset of the flow fields in storeFlows

[eranra]

Allow user configuration to persistent only sub-set of the fields that exist in a flow.

Note 1: This allows efficiency for back-ends. Note 2: Need to discuss read-back of data with limited amount of fields, might create challenges Note 3: Might be also relevant Topology persistence, need to discuss (e.g. in the context of k8s)