Expose a Form::into_stream() method on async multipart forms.

The async equivalent of the change in #2524.

An example use case is compressing multipart form data with zstd.
The entire contents of the payload have to be compressed together,
which requires accessing the contents of the Form.

The existing stream functionality mostly implements what we need,
but just isn't publicly accessible. This PR adds a pub method for it.

src/async_impl/multipart.rs

@@ -140,11 +140,23 @@ impl Form {
     }
 
     /// Consume this instance and transform into an instance of Body for use in a request.
-    pub(crate) fn stream(mut self) -> Body {
+    pub(crate) fn stream(self) -> Body {
         if self.inner.fields.is_empty() {
             return Body::empty();
         }
 
+        Body::stream(self.into_stream())
+    }
+
+    /// Produce a stream of the bytes in this `Form`, consuming it.
+    pub fn into_stream(mut self) -> impl Stream<Item = Result<Bytes, crate::Error>> + Send + Sync {
+        if self.inner.fields.is_empty() {
+            let empty_stream: Pin<
+                Box<dyn Stream<Item = Result<Bytes, crate::Error>> + Send + Sync>,
+            > = Box::pin(futures_util::stream::empty());
+            return empty_stream;
+        }
+
         // create initial part to init reduce chain
         let (name, part) = self.inner.fields.remove(0);
         let start = Box::pin(self.part_stream(name, part))
@@ -161,7 +173,7 @@ impl Form {
         let last = stream::once(future::ready(Ok(
             format!("--{}--\r\n", self.boundary()).into()
         )));
-        Body::stream(stream.chain(last))
+        Box::pin(stream.chain(last))
     }
 
     /// Generate a hyper::Body stream for a single Part instance of a Form request.