Possible useful name pattern for PTH documentation

This is not a solution for every use case, in large part because the Reflections scan can take some time, but it's a pattern that may be generally applicable enough to describe in the documentation:

- Define an annotation for the subtypes, and meta-annotate with `@JsonTypeName`:
```
@JsonTypeName
@Target({ElementType.ANNOTATION_TYPE, ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
public @interface FoocorpJsonType {
}
```

- Use `@FoocorpJsonType` on all subtypes to be registered with the mapper:
```
@FoocorpJsonType
public class ConcreteMessage extends AbstractMessage {}
```

- In a suitable location (such as a `@PostConstruct` method on a Spring configuration class, or wherever the `ObjectMapper` is built), use the `org.reflections:reflections` package and this magic spell:
```
Reflections reflections = new Reflections(
ConfigurationBuilder.build("com.foocorp.base.package")
);

mapper.registerSubtypes(reflections.getTypesAnnotatedWith(FoocorpJsonType.class).toArray(new Class[0]));
```

This will scan the classpath (limited for performance) for types annotated with the custom annotation and register them all with the mapper so they can be deserialized when seen.