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.

packages/fullstack-server/src/redirect.rs

@@ -0,0 +1,31 @@
+use std::sync::OnceLock;
+
+/// A custom header that can be set with any value to indicate
+/// that the server function client should redirect to a new route.
+///
+/// This is useful because it allows returning a value from the request,
+/// while also indicating that a redirect should follow. This cannot be
+/// done with an HTTP `3xx` status code, because the browser will follow
+/// that redirect rather than returning the desired data.
+pub const REDIRECT_HEADER: &str = "serverfnredirect";
+
+/// A function that will be called if a server function returns a `3xx` status
+/// or the [`REDIRECT_HEADER`].
+pub type RedirectHook = Box<dyn Fn(&str) + Send + Sync>;
+
+// allowed: not in a public API, and pretty straightforward
+#[allow(clippy::type_complexity)]
+pub(crate) static REDIRECT_HOOK: OnceLock<RedirectHook> = OnceLock::new();
+
+/// Sets a function that will be called if a server function returns a `3xx` status
+/// or the [`REDIRECT_HEADER`]. Returns `Err(_)` if the hook has already been set.
+pub fn set_redirect_hook(hook: impl Fn(&str) + Send + Sync + 'static) -> Result<(), RedirectHook> {
+    REDIRECT_HOOK.set(Box::new(hook))
+}
+
+/// Calls the hook that has been set by [`set_redirect_hook`] to redirect to `loc`.
+pub fn call_redirect_hook(loc: &str) {
+    if let Some(hook) = REDIRECT_HOOK.get() {
+        hook(loc)
+    }
+}