Split client to async and blocking

[2e3s]

The problem is not only that the current functionality is less useful in async context. The client cannot be used in async context at all as is:
https://docs.rs/reqwest/latest/reqwest/blocking/

Conversely, the functionality in reqwest::blocking must not be executed within an async runtime, or it will panic when attempting to block

I've used tokio's runtime to block because reqwest uses tokio blocking internally as well.
https://github.com/seanmonstar/reqwest/blob/master/src/blocking/wait.rs
It also uses a current thread runtime, but with timeouts. However, the async client specified the timeout already, so it shouldn't matter much.

[2e3s]

This watcher doesn't use rev to depend on a commit or any pin of the kind https://github.com/ActivityWatch/aw-watcher-window-wayland/blob/master/Cargo.toml (which is not good exactly because of this), so it will have to be updated to be built successfully, if this change is accepted.

[2e3s]

One alternative would be "async generic" so to speak: https://docs.rs/maybe-async/0.2.6/maybe_async/ - it would exclude tokio from crate's dependencies (reqwest would use it regardless), maybe increase build time, remove some code duplication.

[ErikBjare]

Hmm, can't figure out why CI doesn't run

[ErikBjare]

CI passed fine in #419, force-pushed the rebased commit here and merging :)