Consider switching ureq for something async (reqwest, hyper)

[joepio]

Anything that fetches something will now block the thread, because I use ureq. We could switch to something like hyper (which btw we're probably depending on pretty soon, anyway #192).

However, introducing async stuff into Atomic-Server warrants at least some more discussion. In issue #12 I basically concluded that we didn't need async, and that it might hurt performance. Async has overhead - that's unavoidable. I'm not sure how bad it is, though.

Also, note that we already have a little bit of async in atomic_lib, in a plugin handler for email.

What happens if we make fetch async

• get_resouce will become async
• this is used practically everywhere, so now pretty much everything will become async.

If async is everywhere, is that a problem? I've read that rust devs find async pretty hard, so that means it might become harder to use atomic_lib.

Anyways, I believe async can't be used in traits right now, but this is probably changing soon, as it's already in nightly since november 2022.