samples: drivers: uart: Add a 'passthrough' example

This sample can be used as a "virtual wire", allowing direct access to
devices connected to the target processor - for example: GPS, Cellular,
etc...

This is also useful as a utility - no other connectivity options exist
for UART, where other interfaces like SPI and I2C have shell commands.

Signed-off-by: Attie Grande <attie.grande@argentum-systems.co.uk>