Feature Request: Metadata field for embargoed datasets

Overview of the Feature Request
The latest survey on the requirements of Open Science Horizon Europe MGA https://zenodo.org/records/7728016 also includes the requirement for a separate embargo field. At the moment this information is only on file level available (JSON metadata export):

"dataFile": {
"id": 3925,
"embargo": {
"dateAvailable": "2024-10-31"

At the moment the oai_datacite and DataCite exports:
• If all files /non-restricted: <rights rightsURI="info:eu-repo/semantics/openAccess"/>
• If at least one file is restricted: <rights rightsURI="info:eu-repo/semantics/restrictedAccess"/>

A good solution would be, if at least one file is under embargo then the oai_datacite and DataCite exports include <rights rightsURI="info:eu-repo/semantics/embargoedAccess"/>, following OpenAIRE's guidelines about Rights at
https://guidelines.openaire.eu/en/latest/data/field_rights.html#rightsuri-ma

What inspired the request?
The latest HE MGA survey

Any open or closed issues related to this feature request?

• Feature Request/Idea: Support compliance with the Open Science Horizon Europe Model Grant Agreement (OS HE MGA) Requirements #10196
• Feature Request/Idea: Provide an "Open Access" badge on a dataset page #9943
• Feature Request/Idea: Make embargoes more visible #9678
• https://github.com/IQSS/dataverse/issues/5920

Start by locating the oai_datacite and DataCite export implementations and reviewing how file-level embargo metadata is currently represented. Trace the existing openAccess and restrictedAccess rights mapping, then determine how to emit embargoedAccess when any file is under embargo while preserving the other cases. Done means both exports follow the stated OpenAIRE rights behavior.