Support State with #[derive(FromRequest[Parts])] (#1391)

* Support `State` with `#[derive(FromRequest[Parts])]`

Fixes https://github.com/tokio-rs/axum/issues/1314

This makes it possible to extract things via `State` in
`#[derive(FromRequet)]`:

```rust
struct Foo {
    state: State<AppState>,
}
```

The state can also be inferred in a lot of cases so you only need to
write:

```rust
struct Foo {
    // since we're using `State<AppState>` we know the state has to be
    // `AppState`
    state: State<AppState>,
}
```

Same for

```rust
struct Foo {
    #[from_request(via(State))]
    state: AppState,
}
```

And

```rust
struct AppState {}
```

I think I've covered all the edge cases but there are (unsurprisingly) a
few.

* make sure things can be combined with other extractors

* main functions in ui tests don't need to be async

* Add test for multiple identicaly state types

* Add failing test for multiple states

axum-macros/tests/from_request/pass/state_enum_via.rs

@@ -0,0 +1,34 @@
+use axum::{
+    extract::{State, FromRef},
+    routing::get,
+    Router,
+};
+use axum_macros::FromRequest;
+
+fn main() {
+    let _: Router<AppState> = Router::with_state(AppState::default())
+        .route("/a", get(|_: AppState| async {}))
+        .route("/b", get(|_: InnerState| async {}));
+}
+
+#[derive(Clone, FromRequest)]
+#[from_request(via(State))]
+enum AppState {
+    One,
+}
+
+impl Default for AppState {
+    fn default() -> AppState {
+        Self::One
+    }
+}
+
+#[derive(FromRequest)]
+#[from_request(via(State), state(AppState))]
+enum InnerState {}
+
+impl FromRef<AppState> for InnerState {
+    fn from_ref(_: &AppState) -> Self {
+        todo!(":shrug:")
+    }
+}