Improving example main.cpp to handle oob() case

Hi, @SenRamakri and @deepikabhavnani

Current example is focused on just sending AT-commands to the target serial end point.
Any chance we enable AT-command receiver example using oob() callback registration?
My important customer wants to reuse Mbed-OS AT-cmd parser for their NB-IoT AT-command server.
In other words, Mbed target board receiving AT-command need to use oob() calls and parse AT-commands.

My very important customer would like to have an example on this feature so that they don't have to reinvent the wheel.

Thanks,