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/docs/request_origin.md

@@ -0,0 +1,48 @@
+This method interacts with information from the current request. The request may come from:
+
+1. The initial SSR render if this method called from a [`Component`](dioxus_core_macro::component) or a [`server`](dioxus_server) function that is called during the initial render
+
+```rust
+# use dioxus::prelude::*;
+#[component]
+fn PrintHtmlRequestInfo() -> Element {
+    // The server context only exists on the server, so we need to put it behind a server_only! config
+    server_only! {
+        // Since we are calling this from a component, the server context that is returned will be from
+        // the html request for ssr rendering
+        let context = server_context();
+        let request_parts = context.request_parts();
+        println!("headers are {:?}", request_parts.headers);
+    }
+    rsx! {}
+}
+```
+
+2. A request to a [`server`](dioxus_server) function called directly from the client (either on desktop/mobile or on the web frontend after the initial render)
+
+```rust
+# use dioxus::prelude::*;
+#[server]
+async fn read_headers() -> ServerFnResult {
+    // Since we are calling this from a server function, the server context that is may be from the
+    // initial request or a request from the client
+    let context = server_context();
+    let request_parts = context.request_parts();
+    println!("headers are {:?}", request_parts.headers);
+    Ok(())
+}
+
+#[component]
+fn CallServerFunction() -> Element {
+    rsx! {
+        button {
+            // If you click the button, the server function will be called and the server context will be
+            // from the client request
+            onclick: move |_| async {
+                _ = read_headers().await
+            },
+            "Call server function"
+        }
+    }
+}
+```