Add example of using Embassy HAL(stm32) with RTIC. (#891)

The RTIC book mentions Embassy+RTIC but gives no examples.

fmt.

Add feature flag

Seems CI does not deal with 2 levels of depth.

Forgot to stage.

Thumb m arch.

Co-authored-by: Corey Schuhen <cschuhen@gmail.com>

examples/embassy-stm32g4/src/bin/blinky.rs

@@ -0,0 +1,61 @@
+#![deny(unsafe_code)]
+#![deny(warnings)]
+#![no_main]
+#![no_std]
+#![feature(type_alias_impl_trait)]
+
+use defmt::*;
+use embassy_stm32::gpio::{Level, Output, Speed};
+use rtic::app;
+use rtic_monotonics::systick::*;
+use {defmt_rtt as _, panic_probe as _};
+
+pub mod pac {
+    pub use embassy_stm32::pac::Interrupt as interrupt;
+    pub use embassy_stm32::pac::*;
+}
+
+#[app(device = pac, peripherals = false, dispatchers = [SPI1])]
+mod app {
+    use super::*;
+
+    #[shared]
+    struct Shared {}
+
+    #[local]
+    struct Local {}
+
+    #[init]
+    fn init(cx: init::Context) -> (Shared, Local) {
+        // Initialize the systick interrupt & obtain the token to prove that we did
+        let systick_mono_token = rtic_monotonics::create_systick_token!();
+        Systick::start(cx.core.SYST, 25_000_000, systick_mono_token);
+
+        let p = embassy_stm32::init(Default::default());
+        info!("Hello World!");
+
+        let mut led = Output::new(p.PC6, Level::High, Speed::Low);
+        info!("high");
+        led.set_high();
+
+        // Schedule the blinking task
+        blink::spawn(led).ok();
+
+        (Shared {}, Local {})
+    }
+
+    #[task()]
+    async fn blink(_cx: blink::Context, mut led: Output<'static, embassy_stm32::peripherals::PC6>) {
+        let mut state = true;
+        loop {
+            info!("blink");
+            if state {
+                led.set_high();
+            } else {
+                led.set_low();
+            }
+            state = !state;
+            Systick::delay(1000.millis()).await;
+        }
+    }
+}