feat(shell): Add commands to configure Modbus

- Implement a new 'modbus' command in the shell.
- Add sub-commands 'set_baud', 'set_id', and 'show'.
- Add validation for baud rate and slave ID inputs.
- The new parameters are applied to the Modbus server at runtime, allowing for live reconfiguration of the communication settings.
- The shell backend is set to RTT.

software/apps/slave_node/boards/bluepill_f103rb.overlay

@@ -1,7 +1,7 @@
 / {
     chosen {
         zephyr,console = &rtt;
-        zephyr,shell-uart = &rtt; // If using shell
+        zephyr,shell = &rtt;
     };
 
     rtt: rtt {