feat(http2): allow configuring the HTTP/2 frame size

The default of 16K is taken from h2.

src/server/conn.rs

@@ -309,6 +309,18 @@ impl<E> Http<E> {
         self
     }
 
+    /// Sets the maximum frame size to use for HTTP2.
+    ///
+    /// Passing `None` will do nothing.
+    ///
+    /// If not set, hyper will use a default.
+    pub fn http2_max_frame_size(&mut self, sz: impl Into<Option<u32>>) -> &mut Self {
+        if let Some(sz) = sz.into() {
+            self.h2_builder.max_frame_size = sz;
+        }
+        self
+    }
+
     /// Sets the [`SETTINGS_MAX_CONCURRENT_STREAMS`][spec] option for HTTP2
     /// connections.
     ///