title: Add Header and Footer for AvroIO Writer
body: <p>We have a requirements to add a header and footer to a output avro file, but seems like the SDK doesn't support it. For TextIO writer, it seems to have that capability <code>withHeader</code> and <code>withFooter</code>.</p>
  <p>That been said, what's the best way to do it without creating a separate pipeline? I tried add another step after the writer, but seems like the pipeline assumed to be ended after the writer.</p>