feat: embassy-usb-logger and example for rpi pico

* Add embassy-usb-logger which allows logging over USB for any device
  implementing embassy-usb
* Add example using logger for rpi pico.

examples/rp/src/bin/usb_logger.rs

@@ -0,0 +1,30 @@
+#![no_std]
+#![no_main]
+#![feature(type_alias_impl_trait)]
+
+use embassy_executor::Spawner;
+use embassy_rp::interrupt;
+use embassy_rp::peripherals::USB;
+use embassy_rp::usb::Driver;
+use embassy_time::{Duration, Timer};
+use {defmt_rtt as _, panic_probe as _};
+
+#[embassy_executor::task]
+async fn logger_task(driver: Driver<'static, USB>) {
+    embassy_usb_logger::run!(1024, log::LevelFilter::Info, driver);
+}
+
+#[embassy_executor::main]
+async fn main(spawner: Spawner) {
+    let p = embassy_rp::init(Default::default());
+    let irq = interrupt::take!(USBCTRL_IRQ);
+    let driver = Driver::new(p.USB, irq);
+    spawner.spawn(logger_task(driver)).unwrap();
+
+    let mut counter = 0;
+    loop {
+        counter += 1;
+        log::info!("Tick {}", counter);
+        Timer::after(Duration::from_secs(1)).await;
+    }
+}