Add #[derive(FromRef)] (#1430)

* add `#[derive(FromRef)]`

* tests

* don't support skipping fields

probably wouldn't work at all since the whole state likely needs `Clone`

* UI tests

* changelog

* changelog link

* revert hello-world example, used for testing

* Re-export `#[derive(FromRef)]`

* Don't need to return `Result`

* use `collect` instead of quoting the iterator

* Mention it in axum's changelog

axum-core/src/extract/from_ref.rs

@@ -5,7 +5,10 @@
 ///
 /// See [`State`] for more details on how library authors should use this trait.
 ///
+/// This trait can be derived using `#[derive(axum_macros::FromRef)]`.
+///
 /// [`State`]: https://docs.rs/axum/0.6/axum/extract/struct.State.html
+/// [`#[derive(axum_macros::FromRef)]`]: https://docs.rs/axum-macros/latest/axum_macros/derive.FromRef.html
 // NOTE: This trait is defined in axum-core, even though it is mainly used with `State` which is
 // defined in axum. That allows crate authors to use it when implementing extractors.
 pub trait FromRef<T> {