Please make the HighlightQuery class public

We're using the highlighter directly in our code, and the introduction of the package-scoped HighlightQuery in 4aa59aff0058b05e822592431eda4a469a6b9eef has made this impossible without building our own little fork.
`protected` makes sense, but it would require moving the instantiation of `HighlightQuery` from [`HighlightPhase`](https://github.com/elasticsearch/elasticsearch/blob/4aa59aff0058b05e822592431eda4a469a6b9eef/src/main/java/org/elasticsearch/search/highlight/HighlightPhase.java#L112) to the `HighlightContext` constructor.

Actually, if we do that, I no longer need `HighlightQuery` to be public at all. It can be private and considered an implementation detail. But this is no longer the simplest thing that works, and actually raises a bunch of other questions.

I created #6460 in case you want to go down that path.