* extra: Add `Cached` extractor
`Cached` wraps another extractor and caches its result in request
extensions.
* Use newtype to avoid overriding extensions of the same type
* Rename type param
* Add `axum-extra` crate
Empty for now but now we have a place to put stuff.
I'll make a PR for moving over https://github.com/davidpdrsn/axum-resource.
* remove `authors` field from `Cargo.toml`s