title: Serializing a nested json object into an opened Utf8JsonWriter
body: <p>While serializing with <code>Utf8JsonWriter</code> want to add a field which is a complex object so I want to just serialize it like a nested json:</p>
  <pre><code>public override void Write(Utf8JsonWriter writer, Foo foo, JsonSerializerOptions options)
  {
      writer.WriteStartObject();
      writer.WriteString(&quot;simpleField1&quot;, &quot;value1&quot;);
      writer.WriteString(&quot;simpleField2&quot;, &quot;value2&quot;);
      ....
      writer.WriteString(&quot;bar&quot;, JsonSerializer.Serialize(foo.Bar));
      ....
      writer.WriteEndObject();
      writer.Flush();
  }
  </code></pre>
  <p>This results in any quotes being escaped to <code>\u0022</code>, however. How can I properly add an already serialized object while writing in <code>Utf8JsonWriter</code>?</p>