Support changing state type in #[debug_handler] (#1271)

* support setting body type for #[debug_handler]

* Use lookahead1 to give better errors and detect duplicate arguments

* fix docs link

axum-macros/tests/debug_handler/pass/set_state.rs

@@ -0,0 +1,27 @@
+use axum_macros::debug_handler;
+use axum::extract::{FromRef, FromRequest, RequestParts};
+use axum::async_trait;
+
+#[debug_handler(state = AppState)]
+async fn handler(_: A) {}
+
+#[derive(Clone)]
+struct AppState;
+
+struct A;
+
+#[async_trait]
+impl<S, B> FromRequest<S, B> for A
+where
+    B: Send,
+    S: Send + Sync,
+    AppState: FromRef<S>,
+{
+    type Rejection = ();
+
+    async fn from_request(_req: &mut RequestParts<S, B>) -> Result<Self, Self::Rejection> {
+        unimplemented!()
+    }
+}
+
+fn main() {}