Create axum-kv

[marvin-j97]

The examples are supposed to show you best practices when using fjall inside a web server, but turns out I'm absolutely terrible at axum!

If an axum master mind comes by, this is your chance.

See actix-kv for inspiration, it has:

• a custom error responder for ? syntax
• unit testing
• web::block calls to prevent blocking the tokio event loop