Improve Http2FrameCodecBuilder usability (#11195)

__Motivation__

 `Http2FrameCodecBuilder` constructor calls `server()` internally which disallows using certain methods on the builder later. Additionally, the constructor is package private which limits extension of the `Builder` as well as usage outside the available `forClient()` and `forServer()` static methods.

 __Modification__

 - Introduce a `public` no-arg constructor to `Http2FrameCodecBuilder`.

 __Result__

 `Http2FrameCodecBuilder` can now be used to create the codec with `Http2Connection` or `Http2ConnectionDecoder` and `Http2ConnectionEncoder` which was earlier not possible due to implicit call to `server()` by the `Http2FrameCodecBuilder` constructor.

codec-http2/src/main/java/io/netty/handler/codec/http2/Http2FrameCodecBuilder.java

@@ -29,6 +29,9 @@ public class Http2FrameCodecBuilder extends
 
     private Http2FrameWriter frameWriter;
 
+    public Http2FrameCodecBuilder() {
+    }
+
     Http2FrameCodecBuilder(boolean server) {
         server(server);
         // For backwards compatibility we should disable to timeout by default at this layer.