[#311] services/control: Implement NetmapSnapshot method

Implement methods required for signatures. Receive network map from netmap
storage, convert it to Control service message and return in response.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

cmd/neofs-node/control.go

@@ -42,6 +42,7 @@ func initControlService(c *cfg) {
 		controlSvc.WithKey(c.key),
 		controlSvc.WithAuthorizedKeys(keys),
 		controlSvc.WithHealthChecker(c),
+		controlSvc.WithNetMapSource(c.cfgNetmap.wrapper),
 	)
 
 	var (