Explore the idea to provide non-async APIs

[m4tx]

While async bring some nice features and makes it easy to create code that "does a few things at once", it's generally more difficult to write, reason about, and debug.

It might be useful to consider providing a non-async APIs that would be used by default, and make async an option that you need to consciously decide to use.

A more lightweight alternative would be to remove the Send + 'static bound on the request handlers. This can be done with pure hyper (no axum): https://emschwartz.me/async-rust-can-be-a-pleasure-to-work-with-without-send-sync-static/