Added tags aggregation in SearchController for .NET Core example

[rdcm]

No description provided.

[russcam]

Thanks for opening this PR @rdcm!

Before this can be merged, the README walkthrough would also need to be updated to reflect the changes within the code, since the two are intended to be used together.

Is this something that you would be able to address?

[rdcm]

@russcam i take this code from branch
https://github.com/elastic/elasticsearch-net-example/blob/5.x-codecomplete-netcore-modifications/src/NuSearch.Web/Modules/Search/Search/SearchController.cs

with little bit refactoring.

Readme in 5.x-codecomplete-netcore-modifications branch not reflect the real code in controller, not contains Apply.. methods and tags aggregations description. I'm not sure I can describe it well. But without this feature sample does not look complete.