Add GRPCServerFlushNotificationHandler (#1839)

* Add GRPCServerFlushNotificationHandler

* Change channel outbound handler required typealiases to Any

Sources/GRPCHTTP2Core/Server/Connection/GRPCServerFlushNotificationHandler.swift

@@ -0,0 +1,36 @@
+/*
+ * Copyright 2024, gRPC Authors All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import NIOCore
+
+@available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *)
+final class GRPCServerFlushNotificationHandler: ChannelOutboundHandler {
+  typealias OutboundIn = Any
+  typealias OutboundOut = Any
+
+  private let serverConnectionManagementHandler: ServerConnectionManagementHandler
+
+  init(
+    serverConnectionManagementHandler: ServerConnectionManagementHandler
+  ) {
+    self.serverConnectionManagementHandler = serverConnectionManagementHandler
+  }
+
+  func flush(context: ChannelHandlerContext) {
+    self.serverConnectionManagementHandler.syncView.connectionWillFlush()
+    context.flush()
+  }
+}