feat(examples): add usb-serial example

[kaspar030]

Description

This adds a usb serial example.
It is basically a port of one of Embassy's examples.

I tested that it works, with Linux as host, on nrf52840dk, rpi-pico and two stm32 (which are not in main yet).

Issues/PRs references

Open Questions

Change checklist

• I have cleaned up my commit history and squashed fixup commits.
• I have followed the Coding Conventions.
• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.

[ROMemories]

Maybe we could print/log the data rate?

---

The following

info!("Data rate: {} bauds", class.line_coding().data_rate());

gives me 8000 bauds.

[kaspar030]

gives me 8000 bauds.

I digged a bit, IMO that can just be ignored.

upstream code says so. Also, the baud rate can only be set from the host.