Allow DataObject properties to be ignored by generated converter

In certain circumstances it is desirable or necessary to omit a property from the generated converter.

For example, the following Kotlin DataObject will not compile successfully:

```kotlin
@DataObject(generateConverter = true)
class EnumSetDataObject() {
var enumSet: Set = emptySet()
...
}

error: Could not generate element for io.vertx.example.EnumSetDataObject:
Illegal type ? extends io.vertx.example.MyEnum of kind WILDCARD
```

The desired effect (omitting `enumSet` from the converter) can be accomplished in Kotlin by using the `@JvmField` annotation, however, that is a language-specific solution.

I think that it would be useful to provide this functionality, either via the `@GenIgnore` annotation or a new `@ConverterIgnore` annotation.