crossrefutils: new Fundref-based APIs

• Introduces new query_fundref_api() and get_all_modified_dois() functions, namely needed by the SCOAP3 project, allowing to query the Fundref-based Crossref APIs to discover when new DOIs are registered by given publishers. This indirectly allows to approximately know when they are first created, an information that is not available through the regular Crossref APIs.

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>