implement persistent configuration database
extend the gnmi target to contain a persistent configdb in which configured leafs are saved (system target state)
this configdb can then be used to answer gnmi requests without checking the actual os configuration, but only if the os client keeps it updated via polling or subscription and appropriately reacts to changes
part of this issue is to decide how exactly this configdb is implemented (database? config files? viper?) - possibly talk to Andre Sterba since he implemented persistence in gosdn