5x search result export metadata

https://jira.duraspace.org/browse/DS-1262

Sorry for the previous bad design attempts at this feature. In the future I will test and confirm correctness before submitting pull request. I hope this has minimal mistakes and meets the requirements of the JIRA ticket.
In this case that may not be possible, at least without great measures. The code was duplicated from performSearch method that is part of the abstract class AbstractSearch. The performSearch method is used to return a cached results page of a search from the complete results returned from solr. I wanted the metadata exported to be as similar to the search results as possible. That is why I included the grouping.