example(grand_central_m4): Added new async eic example

[supersimple33]

Summary

I added a new async example for External Interrupt Controller.

Checklist

• All new or modified code is well documented, especially public items
• No new warnings or clippy suggestions have been introduced - CI will deny clippy warnings by default! You may #[allow] certain lints where reasonable, but ideally justify those with a short comment.