Add minimal example: hello-mailbox-async

[CernyMil]

Description

A clear and concise description of the changes.

Related Issue

• Closes # (if applicable)

Checklist

• I have read the CONTRIBUTING.md document.
• My code follows the project's coding standards.
• I have added tests to cover my changes.
• All new and existing tests passed (make check).
• I have updated the documentation accordingly.

[lxsaah]

Thanks @CernyMil. The example is clear and the Mailbox latest-wins demo works as intended. I checked it out locally and cargo run -p hello-mailbox-async produces the expected Got: Blue output. A few inline comments before this can merge. The blocker is the embassy = "0.0.0" line in aimdb-core/Cargo.toml: that's the cause of the could not compile embassy errors from #99 and it will fail the embedded/wasm CI matrix here, so it needs to be removed. Beyond that, please move the in-closure sleeps to the runtime timer and finish the Makefile wiring from the issue checklist. Once those are addressed this should be ready.