The shell sample, and a two-board rig for testing on hardware

samples/shell is `rb9704` on the shell with callbacks that print what
arrives, a CRC-32 and the start of each message from the network. Board
overlays for native_sim (the modem a host serial port, a pty or a real
9704 on USB), NUCLEO-U575ZI-Q, XIAO ESP32S3 and ESP32-S3-DevKitC.

tests/rig/bridge copies bytes between a board's USB serial port and a
UART, so two boards wired together stand in for a USB-serial adapter: one
runs the sample, the other bridges its modem UART to the host, where the
fake modem answers. Overlays for a Nucleo and a XIAO, either way round.

tests/rig/bridge/CMakeLists.txt

@@ -0,0 +1,5 @@
+# SPDX-License-Identifier: MIT
+cmake_minimum_required(VERSION 3.20.0)
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(rb9704_bridge)
+target_sources(app PRIVATE src/main.c)