Cast parent builder to this builder return type

[nielspardon]

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This PR adds wrapper methods to the builder class calling the withXXX builder method of the parent builder and casting the result to the class of the current builder, e.g.:

public static abstract class ChildBuilderBase<T extends Child> extends Parent.ParentBuilderBase<T> {
  ...
  @Override
  public Child.ChildBuilderBase<T> withTimestamp(Instant timestamp) {
    return ((Child.ChildBuilderBase<T> ) super.withTimestamp(timestamp));
  }
  ...
}

This allows to actually use inherited builders with a fluent API.

Fixes https://github.com/joelittlejohn/jsonschema2pojo/issues/1242

[joelittlejohn]

Looks like there is some problem with the way this interacts with the 'inner builder' option.

[nielspardon]

Looks like there is some problem with the way this interacts with the 'inner builder' option.

Sorry for that. I think I found a better solution that avoids this issue.