codify "platform" into resolution logic, server-fn/fullstack overhaul (#4596)

overhaul dioxus fullstack using a new implementation of server_fn

This PR is a complete rewrite of dioxus fullstack designed to improve compatibility with axum and to fix a number of compilation errors based on feature flag unification plumbing through macros.

examples/07-fullstack/header_map.rs

@@ -0,0 +1,36 @@
+//! This example shows how you can extract a HeaderMap from requests to read custom headers.
+//!
+//! The extra arguments in the `#[get(...)]` macro are passed to the underlying axum handler,
+//! and only visible on the server. This lets you run normal axum extractors like `HeaderMap`,
+//! `TypedHeader`, `Query`, etc.
+//!
+//! Note that headers returned by the server are not always visible to the client due to CORS.
+//! Headers like `Set-Cookie` are hidden by default, and need to be explicitly allowed
+//! by the server using the `Access-Control-Expose-Headers` header (which dioxus-fullstack does not
+//! currently expose directly).
+
+use dioxus::prelude::*;
+
+fn main() {
+    dioxus::launch(app);
+}
+
+fn app() -> Element {
+    let mut headers = use_action(get_headers);
+
+    rsx! {
+        h1 { "Header Map Example" }
+        button { onclick: move |_| headers.call(), "Get Headers" }
+        if let Some(Ok(headers)) = headers.result() {
+            p { "Response from server:" }
+            pre { "{headers}" }
+        } else {
+            p { "No headers yet" }
+        }
+    }
+}
+
+#[get("/api/example", headers: dioxus::fullstack::HeaderMap)]
+async fn get_headers() -> Result<String> {
+    Ok(format!("{:#?}", headers))
+}