Fix fetching the correct sort field for relations

[kskaiser]

Description

Having a relation defined in discovery.xml including a sort configuration, the Solr query might fail and no results are provided. This happens, if the metadata field used for sorting is a multi-valued field (e.g., "dc.date.issued").
Instead of adding the plain metadata field, the field type must be considered to get the correct Solr field used for sorting.

References

• Fixes Loading a relation fails with a configured sort field #547

Instructions for Reviewers

• Steps to reproduce the behavior:

In discovery.xml in bean "relationAuthorResearchOutputsConfiguration" in the "searchSortConfiguration" set the defaultSortField to "sortDateIssuedDesc", add this sortfield also to the list of sortFields
Then load a researcher profile with the publications. There will not be any results; the layout of the page seems to be of DSpace Core.

List of changes in this PR:

• First, set the sortField by considering the field type

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[kskaiser]

@AdamF42 can you please take a look at this?