Add embassy-lora crate

This crate contains async radio drivers for various lora drivers that
work with embassy timers. The code is imported from Drogue Device (
https://github.com/drogue-iot/drogue-device)

The radio drivers integrate with the async LoRaWAN MAC layer in the
lorawan-device crate.

Also added is an example for the STM32WL55 and for STM32L0 (requires
the LoRa Discovery board) for LoRaWAN. Future work is to make the
underlying radio drivers using fully async SPI when communicating
with the peripheral.

embassy-lora/src/lib.rs

@@ -0,0 +1,23 @@
+#![no_std]
+#![feature(type_alias_impl_trait)]
+#![feature(generic_associated_types)]
+//! embassy-lora is a collection of async radio drivers that integrate with the lorawan-device
+//! crate's async LoRaWAN MAC implementation.
+
+pub(crate) mod fmt;
+
+#[cfg(feature = "stm32wl")]
+pub mod stm32wl;
+#[cfg(feature = "sx127x")]
+pub mod sx127x;
+
+/// A convenience timer to use with the LoRaWAN crate
+pub struct LoraTimer;
+
+#[cfg(feature = "time")]
+impl lorawan_device::async_device::radio::Timer for LoraTimer {
+    type DelayFuture<'m> = impl core::future::Future<Output = ()> + 'm;
+    fn delay_ms<'m>(&'m mut self, millis: u64) -> Self::DelayFuture<'m> {
+        embassy::time::Timer::after(embassy::time::Duration::from_millis(millis))
+    }
+}