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/08-apis/custom_html.rs

@@ -0,0 +1,34 @@
+//! This example shows how to use a custom index.html and custom <HEAD> extensions
+//! to add things like stylesheets, scripts, and third-party JS libraries.
+
+use dioxus::prelude::*;
+
+fn main() {
+    dioxus::LaunchBuilder::new()
+        .with_cfg(
+            dioxus::desktop::Config::new().with_custom_index(
+                r#"
+<!DOCTYPE html>
+<html>
+  <head>
+    <title>Dioxus app</title>
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <style>body { background-color: olive; }</style>
+  </head>
+  <body>
+    <h1>External HTML</h1>
+    <div id="main"></div>
+  </body>
+</html>
+        "#
+                .into(),
+            ),
+        )
+        .launch(app);
+}
+
+fn app() -> Element {
+    rsx! {
+        h1 { "Custom HTML!" }
+    }
+}