[SYS-1015] Allocate IOStreamWithOwnedBuf with Aws::New

Summary: The factory needs to create the stream with Aws::New instead of new, as per https://docs.aws.amazon.com/sdk-for-cpp/v1/developer-guide/configuring-iostreams.html

Test Plan: make check

Reviewers: tudor, ngbronson, dhruba

Differential Revision: https://rockset.phacility.com/D13256

cloud/aws/aws_s3.cc

@@ -936,7 +936,8 @@ Status S3StorageProvider::DoGetCloudObject(const std::string& bucket_name,
             Aws::Utils::ARRAY_ALLOCATION_TAG, destination,
             std::ios_base::out | std::ios_base::trunc);
       }
-      return new IOStreamWithOwnedBuf<WritableFileStreamBuf>(
+      return Aws::New<IOStreamWithOwnedBuf<WritableFileStreamBuf>>(
+          Aws::Utils::ARRAY_ALLOCATION_TAG,
           std::unique_ptr<WritableFileStreamBuf>(new WritableFileStreamBuf(
               std::unique_ptr<WritableFileWriter>(new WritableFileWriter(
                   std::move(file), destination, foptions)))));