rust: add payload trait in message mod

Signed-off-by: Nicolas Buffon <[email protected]>

rust/src/exchange/message.rs

@@ -7,4 +7,6 @@
 // Author: Nicolas BUFFON <[email protected]> et al.
 // Software description: This Intelligent Transportation Systems (ITS) [MQTT](https://mqtt.org/) client based on the [JSon](https://www.json.org) [ETSI](https://www.etsi.org/committee/its) specification transcription provides a ready to connect project for the mobility (connected and autonomous vehicles, road side units, vulnerable road users,...).
 
+pub(crate) mod payload;
+
 enum Message {}

rust/src/exchange/message/payload.rs

@@ -0,0 +1,18 @@
+// Software Name: its-client
+// SPDX-FileCopyrightText: Copyright (c) 2016-2022 Orange
+// SPDX-License-Identifier: MIT License
+//
+// This software is distributed under the MIT license, see LICENSE.txt file for more details.
+//
+// Author: Nicolas BUFFON <[email protected]> et al.
+// Software description: This Intelligent Transportation Systems (ITS) [MQTT](https://mqtt.org/) client based on the [JSon](https://www.json.org) [ETSI](https://www.etsi.org/committee/its) specification transcription provides a ready to connect project for the mobility (connected and autonomous vehicles, road side units, vulnerable road users,...).
+
+use crate::mobility::mobile::Mobile;
+
+pub trait Payload {
+    fn get_type(&self) -> &str;
+
+    fn appropriate(&mut self);
+
+    fn as_mobile(&self) -> Result<&dyn Mobile, &'static str>;
+}