samples: usb: add a cdc_acm_bridge sample

Add a sample for the UART bridge driver using a USB CDC-ACM UART and a
normal one. Demonstrates how to do settings propagation (bitrate
changes) and multiple instances.

The sample needs to know what specific UART are available on each board,
so unfortunately any additional board needs its own overlay.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>