Expose the Zephyr shell via the device protocol

[mondalaci]

Then, Agent would make the shell accessible via xterm.js

https://devzone.nordicsemi.com/f/nordic-q-a/86501/zephyr-shell-with-custom-interface-transport-layer/361292

The link protocol would transfer the shell of the left half.

We could use a new USB protocol command for reading and writing the shell. When there is shell content to read, GetDeviceState would mark the relevant bits as dirty.