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/multipart_form.rs

@@ -0,0 +1,64 @@
+//! This example showcases how to handle multipart form data uploads in Dioxus.
+//!
+//! Dioxus provides the `MultipartFormData` type to allow converting from the websys `FormData`
+//! type directly into a streaming multipart form data handler.
+
+use dioxus::{fullstack::MultipartFormData, prelude::*};
+
+fn main() {
+    dioxus::launch(app);
+}
+
+fn app() -> Element {
+    // The `MultipartFormData` type can be used to handle multipart form data uploads.
+    // We can convert into it by using `.into()` on the `FormEvent`'s data, or by crafting
+    // a `MultipartFormData` instance manually.
+    let mut upload_as_multipart = use_action(move |event: FormEvent| upload(event.into()));
+
+    rsx! {
+        Stylesheet { href: asset!("/examples/assets/file_upload.css") }
+        img { src: asset!("/examples/assets/logo.png"), width: "200px" }
+        div {
+            h3 { "Upload as Multipart" }
+            p { "Use the built-in multipart form handling" }
+            form {
+                display: "flex",
+                flex_direction: "column",
+                gap: "8px",
+                onsubmit: move |evt| async move {
+                    evt.prevent_default();
+                    upload_as_multipart.call(evt).await;
+                },
+                label { r#for: "headshot", "Photos" }
+                input { r#type: "file", name: "headshot", multiple: true, accept: ".png,.jpg,.jpeg" }
+                label { r#for: "resume", "Resume" }
+                input { r#type: "file", name: "resume", multiple: false, accept: ".pdf" }
+                label { r#for: "name", "Name" }
+                input { r#type: "text", name: "name", placeholder: "Name" }
+                label { r#for: "age", "Age" }
+                input { r#type: "number", name: "age", placeholder: "Age" }
+                input { r#type: "submit", name: "submit", value: "Submit your resume" }
+            }
+        }
+    }
+}
+
+/// Upload a form as multipart form data.
+///
+/// MultipartFormData is typed over the form data structure, allowing us to extract
+/// both files and other form fields in a type-safe manner.
+///
+/// On the server, we have access to axum's `Multipart` extractor
+#[post("/api/upload-multipart")]
+async fn upload(mut form: MultipartFormData) -> Result<()> {
+    while let Ok(Some(field)) = form.next_field().await {
+        info!(
+            "Field name: {:?}, filename: {:?}, content_type: {:?}",
+            field.name().to_owned(),
+            field.file_name(),
+            field.content_type(),
+        );
+    }
+
+    Ok(())
+}