Add support for USB logging on peripheral

[tompre]

Adds usb init function and logging task for the peripheral depending on the usb_log feature flag.
Should this be a separate feature?

Currently this works for nrf52 only, should I add support for rp2040?

Currently peripheral usb logging is mutually exclusive with dfu, should I create a common usb_task for the two? Would that even work with the available hardware resources?

It changes the default log level to trace, so one can change the log level without touching rmk code. I added docs for this.

Let me know if I should change anything.

[HaoboGu]

Thanks for the work! Comments from me:

1. The default log level should be set to info/debug? trace is quite verbose sometimes
2. Yes, it would be great to have it on rp2040
3. The USB instance can be shared, USB-DFU and logging are different endpoints, so they can co-exist.