Support for trailing headers

[Albertoimpl]

As of today, there is no way to pass through trailing headers into any response, because of that Http2StreamFrameToHttpObjectCodec will not be able, when encoding the last content, to send the End of stream.

This is a problem when using gRPC since the protocol relies on trailing headers.

Considered alternatives

Not really an alternative but, adding a new DefaultHttp2HeadersFrame when markSentBody, will lead to the right code-path.

diff --git a/reactor-netty-http/src/main/java/reactor/netty/http/server/HttpServerOperations.java b/reactor-netty-http/src/main/java/reactor/netty/http/server/HttpServerOperations.java
index f48a2276..6490447c 100644
--- a/reactor-netty-http/src/main/java/reactor/netty/http/server/HttpServerOperations.java
+++ b/reactor-netty-http/src/main/java/reactor/netty/http/server/HttpServerOperations.java
@@ -57,6 +57,8 @@ import io.netty.handler.codec.http.multipart.HttpData;
 import io.netty.handler.codec.http.multipart.HttpPostRequestDecoder;
 import io.netty.handler.codec.http.websocketx.CloseWebSocketFrame;
 import io.netty.handler.codec.http.websocketx.WebSocketCloseStatus;
+import io.netty.handler.codec.http2.DefaultHttp2Headers;
+import io.netty.handler.codec.http2.DefaultHttp2HeadersFrame;
 import io.netty.handler.codec.http2.HttpConversionUtil;
 import io.netty.util.AsciiString;
 import io.netty.util.ReferenceCountUtil;
@@ -629,7 +631,10 @@ class HttpServerOperations extends HttpOperations<HttpServerRequest, HttpServerR
                        f = channel().writeAndFlush(newFullBodyMessage(EMPTY_BUFFER));
                }
                else if (markSentBody()) {
-                       f = channel().writeAndFlush(LastHttpContent.EMPTY_LAST_CONTENT);
+//                     f = channel().writeAndFlush(LastHttpContent.EMPTY_LAST_CONTENT);
+                       f = channel()
+                                       .writeAndFlush(new DefaultHttp2HeadersFrame(new DefaultHttp2Headers(false)
+                                                       .add("grpc-status", "0"), true));
                }
                else {
                        discard();

Additional context

https://github.com/netty/netty/blob/201f6b6bb2dcdff07df6e30552b96fedbcf21759/codec-http2/src/main/java/io/netty/handler/codec/http2/Http2StreamFrameToHttpObjectCodec.java#L134-L137