Json.MixIn (jsonb 1.3-RC1 API)

```
@Retention(CLASS)
@Target(TYPE)
public static @interface Json.MixIn
```
Mark this Class as a MixIn Type that can add Jsonb Annotations on the specified type.
These types are typically in an external project / dependency or otherwise types that we can't or don't want to explicitly annotate with @Json.
In the example below, the VehicleMixin class augments the the generated Vehicle JsonB adapter to use "ford-type" as the json property.
```
{@code
```

- Required Element Summary
  
  Required Elements
  Modifier and Type Required Element Description
  
  Class<?> value
  The concrete type to mix.

- Element Detail
  - value
```
Class<?> value
```
    The concrete type to mix.