Implement end-to-end app payload encryption.

This implements end-to-end encryption between the end-device and
end-application. The encrypted AppSKey or SessionKeyID is forwarded to
the end-application which should be able to decrypt or request the
AppSKey to decrypt the uplink payload. As well the end-application will
be able to enqueue encrypted application payloads.

Using this mechanism, ChirpStack will never have access to the uplink
and downlink application-payloads.

api/rust/src/integration.rs

@@ -31,6 +31,7 @@ impl Into<String> for LogCode {
             LogCode::UplinkFCntRetransmission => "UPLINK_F_CNT_RETRANSMISSION",
             LogCode::DownlinkGateway => "DOWNLINK_GATEWAY",
             LogCode::RelayNewEndDevice => "RELAY_NEW_END_DEVICE",
+            LogCode::FCntDown => "F_CNT_DOWN",
         }
         .to_string()
     }