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/fail/duplicate_args.rs

@@ -0,0 +1,9 @@
+use axum_macros::debug_handler;
+
+#[debug_handler(body = BoxBody, body = BoxBody)]
+async fn handler() {}
+
+#[debug_handler(state = (), state = ())]
+async fn handler_2() {}
+
+fn main() {}