This attempts to do to SpanTermQueryBuilder what has already been changed for TermQueryBuilder. The commit tries to avoid code duplication where possible by pulling what is the same for both QueryBuilders and tests into separate classes.
Relates to #10217
We have some builders, specifically query builders, `SearchSourceBuilder`, `QuerySourceBuilder` and `SuggestBuilder`, that implement `ToXContent` and also allow to build their content as bytes by simply creating a `BytesReference` that holds their json (or yaml etc.) content (`buildAsBytes` methods). They can also print out their content through `toString`. Made sure that those common methods are in one single place and reused where needed.
Also, merged `QueryBuilder` and `BaseQueryBuilder` and made `QueryBuilder` an abstract class instead of an interface.
Closes#11063
