Add endpoint to access published literature list contents

This MR adds an endpoint to access published literature list contents, without the need of having to communicate with SimComp. Supported types include datasets, papers and software. As it stands, this endpoint only supports fetching the contents of published literature lists. However, it might make sense to also support "head" literature lists, providing a unified way of accessing the contents (but note that this introduces a slight overhead, as we need to fetch the literature list for every request). The updated endpoint could look as follows: /api/literature-lists/{literatureListId}/contents/{contentId}