Failed demo for retry: we can't retry a future directly

[Xuanwo]

This is a failed demo for retry implementation.

However, submitting as a PR for a record is still valuable. I learned a lot during implementing it.

It's useful for users who want to build general utils on all futures/async functions.

---

• Good: This demo works on all futures that return Result, including async functions like async fn test_fn() -> Result<()>
• Good: This demo showed up that we can add new features via adding new traits.
• Bad: This demo won't work as we can't retry the existing future.
  • Possible solution: Make the future Clone (doesn't make sense)

[Xuanwo]

This PR is just for the record, and I confirmed that it won't work.

Let's close it.