title: System.Text.Json - The JSON value could not be converted to System.String
body: <p>I have the following <code>json</code> being sent to api</p>
  
  <pre><code>{
       "Id":22,
       "UserId":22,
       "Payload":{
          "Field":"some payload value"
          ...more unknown field/values go here
       },
       "ContextTypeId":1,
       "EventTypeId":1,
       "SessionId":1
  }
  </code></pre>
  
  <p>I would like to map it to the following:</p>
  
  <pre><code>  public class CreateTrackItem : IRequest&lt;int&gt;
      {  
          public int Id { get; set; }
          public int UserId { get; set; }
  
          public string Payload { get; set; }
          public int ContextTypeId { get; set; }
          public int SessionId { get; set; }
          public int EventTypeId { get; set; }
      }
  </code></pre>
  
  <p>When mapped the <code>Payload</code> property fails that it cannot map <code>json</code> to string, i simply want the <code>Payload</code> to be a <code>string</code> version of the <code>json</code> (will go into a <code>jsonb</code> field in <code>postgres</code>)</p>
  
  <p>I am using .NET Core 3.0 and prefer to use the built in <code>System.Text.Json</code> if possible before switching to <code>Newtonsoft</code>.</p>