Add PDF links for IEEE

For IEEE papers, the PDF link cannot be composed from the DOI.

It would require to fetch the meta data from CrossRef (example: https://api.crossref.org/works/10.1109/ASE56229.2023.00214) and use message->link->*->URL for which message->link->*->content-version == "vor" ("version of record").

(This would also work for Springer and ACM. So perhaps we can remove the hard-coded link construction.)