* 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
