actix-multipart: Feature: Add typed multipart form extractor (#2883)

Co-authored-by: Rob Ede <robjtede@icloud.com>

actix-multipart-derive/tests/trybuild/deny-unknown.rs

@@ -0,0 +1,16 @@
+use actix_web::{web, App, Responder};
+
+use actix_multipart::form::MultipartForm;
+
+#[derive(MultipartForm)]
+#[multipart(deny_unknown_fields)]
+struct Form {}
+
+async fn handler(_form: MultipartForm<Form>) -> impl Responder {
+    "Hello World!"
+}
+
+#[actix_web::main]
+async fn main() {
+    App::new().default_service(web::to(handler));
+}