[DSpace 8 Admin Reports] Move part of the code to a full-featured DataService.

[pilasou]

In the component classes of DSpace 8 reports (filtered-collections-component.ts and filtered-items-component.ts), a DSpaceRestService insance is used to send requests to the REST layer. It may be better to create and use a specific data service similar to CollectionDataService.

It is part of the Angular PR for DSpace 8 reports: https://github.com/DSpace/dspace-angular/pull/2163