Improve Java HTTP client integration

Use buffers as-is without copying, add support for direct buffers to
Jackson codec, improve HTTP header wrapper to use underlying data more,
improve how we send data to Java client by adding a small adapter to
avoid double-wrapping the publisher

Author: mtdowling

aws/client/aws-client-awsjson/src/main/java/software/amazon/smithy/java/aws/client/awsjson/AwsJsonProtocol.java

@@ -141,6 +141,6 @@ private EventDecoderFactory<AwsEventFrame> getEventDecoderFactory(ApiOperation<?
     private static DataStream bodyDataStream(HttpResponse response) {
         var contentType = response.headers().contentType();
         var contentLength = response.headers().contentLength();
-        return DataStream.withMetadata(response.body(), contentType, contentLength, null);
+        return DataStream.withMetadata(response.body(), contentType, contentLength == null ? -1 : contentLength);
     }
 }

client/client-http/src/main/java/software/amazon/smithy/java/client/http/DataStreamBodyPublisher.java

@@ -0,0 +1,32 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package software.amazon.smithy.java.client.http;
+
+import java.net.http.HttpRequest;
+import java.nio.ByteBuffer;
+import java.util.concurrent.Flow;
+import software.amazon.smithy.java.io.datastream.DataStream;
+
+/**
+ * Adapts a Smithy {@link DataStream} to the JDK HttpClient's {@link HttpRequest.BodyPublisher} contract.
+ */
+final class DataStreamBodyPublisher implements HttpRequest.BodyPublisher {
+    private final DataStream dataStream;
+
+    DataStreamBodyPublisher(DataStream dataStream) {
+        this.dataStream = dataStream;
+    }
+
+    @Override
+    public long contentLength() {
+        return dataStream.contentLength();
+    }
+
+    @Override
+    public void subscribe(Flow.Subscriber<? super ByteBuffer> subscriber) {
+        dataStream.subscribe(subscriber);
+    }
+}