feat: add Ariel OS support

[kaspar030]

This PR adds Ariel OS integration to embedded-test.

Ariel is a bit different from regular bare-metal targets:

1. It takes care of a lot of system setup (logging, network, panic handler...)
2. It provides threads alongside async tasks

This is reflected in the integration, e.g., non-async tests are run in a thread.

We have an example on how to use this, and a book chapter with more info.

I've tried to find generic solutions here instead of a blanket "ariel-os" feature, but it became messy. So this is what we're using now.

[t-moe]

Hey @kaspar030,

Thanks for your contribution! I wasn't familiar with Ariel OS before, but it looks quite interesting.

I've left a few minor suggestions. Could you also update the feature documentation in README.md and add an entry to the changelog (under "Unreleased")? I'll handle the crate version bumps.

Appreciate it!

[kaspar030]

Thanks!