[#644] Support keepalive during listing

Send whitespaces every time as new object in list is ready
to prevent client from context cancelling.

Signed-off-by: Denis Kirillov <d.kirillov@yadro.com>

api/handler/api.go

@@ -42,6 +42,7 @@ type (
 		RetryMaxBackoff() time.Duration
 		RetryStrategy() RetryStrategy
 		TLSTerminationHeader() string
+		ListingKeepaliveThrottle() time.Duration
 	}
 
 	FrostFSID interface {