title: System.Text.Json Constructor and immutable type
body: <p>I have a lot of immutable DTOs which I need to deserialize from JSON.  Json.NET automatically uses a public constructor should it find one.  However, System.Text.Json seems to require a [JsonConstructor] attribute (even if there is only one constructor).  I am unable to add this to all my classes/structs.  Is there an easy solution?</p>
  <p>e.g. Deserializes fine:</p>
  <pre><code>public readonly struct Model
  {
      public string Value { get; }
  
      [JsonConstructor]
      public Model(string value)
      {
          Value = value
      }
  }
  </code></pre>
  <p>Does not work:</p>
  <pre><code>public readonly struct Model
  {
      public string Value { get; }
  
      public Model(string value)
      {
          Value = value
      }
  }
  </code></pre>