Add support for aggregations in search result

[tomasz-grobelny]

No description provided.

[ArtificialOwl]

So ...

I am working on public interface to integrate fulltextsearch to the core, I am adding

public function addAggregation(string $category, string $value, int $count): ISearchResult;
public function getAggregations(string $category): array;

to the ISearchResult interface, we will check later to implement your pulls to the fts and fts_elasticsearch when the integration into core is done.