feat(http2): Add window size config options for Client and Server

Add `fn http2_initial_stream_window_size` and `fn
http2_initial_connection_window_size` for client and server.

Closes #1771

src/client/mod.rs

@@ -971,6 +971,25 @@ impl Builder {
         self
     }
 
+    /// Sets the [`SETTINGS_INITIAL_WINDOW_SIZE`][spec] option for HTTP2
+    /// stream-level flow control.
+    ///
+    /// Default is 65,535
+    ///
+    /// [spec]: https://http2.github.io/http2-spec/#SETTINGS_INITIAL_WINDOW_SIZE
+    pub fn http2_initial_stream_window_size(&mut self, sz: impl Into<Option<u32>>) -> &mut Self {
+        self.conn_builder.http2_initial_stream_window_size(sz.into());
+        self
+    }
+
+    /// Sets the max connection-level flow control for HTTP2
+    ///
+    /// Default is 65,535
+    pub fn http2_initial_connection_window_size(&mut self, sz: impl Into<Option<u32>>) -> &mut Self {
+        self.conn_builder.http2_initial_connection_window_size(sz.into());
+        self
+    }
+
     /// Sets the maximum idle connection per host allowed in the pool.
     ///
     /// Default is `usize::MAX` (no limit).