api: Rename time to gw_time. Add ns_time.

The gw_time defines the RX time by the gateway, the ns_time defines when
it was received by the NS. The latter could for example help to debug
latency between the GW <> NS.

api/rust/src/gw.rs

@@ -117,7 +117,8 @@ impl UplinkFrame {
                 self.rx_info = Some(UplinkRxInfo {
                     gateway_id: hex::encode(&rx_info.gateway_id),
                     uplink_id: rng.gen::<u32>(),
-                    time: rx_info.time.clone(),
+                    gw_time: rx_info.time.clone(),
+                    ns_time: None,
                     time_since_gps_epoch: rx_info.time_since_gps_epoch.clone(),
                     fine_time_since_gps_epoch: None,
                     rssi: rx_info.rssi,