Enable field sorting

[patrick-austin]

Currently we do not allow for the Lucene search results to be sorted explicitly, and only rely on the default sorting by score. In principle, we could sort on any indexed field, but dates are likely the most useful case here.

Lucene component should be capable of accepting a specified field to sort on a part of a search request. Being able to sort by fields is relevant to ral-facilities/datagateway#1152