refactor: use async fns in minimal example

[onbjerg]

Converts the minimal example to showcases that an ExEx is just async fn(ctx) -> future and that you can use async fns for both, allowing you to construct ExEx's without implementing Future manually

[DaniPopes]

Can we convert the other examples to use async fn instead of impl Future? This is much more readable

[gakonst]

totally agree with dani, unless there's some non obvious advantage here.

does this also mean we can do 2 exex's that are tokio::join'ed?

[onbjerg]

I think if we want to convert it all to async fns, I'm going to scrap async-fn and just convert minimal to an async fn, then in follow ups convert the rest

[DaniPopes]

sgtm

[shekhirin]

bullish