Add DOI to Upload

Closes #2540 (closed)

• Generalize DOI handling (dataset, upload)
• Strip down DOI class (only to be used for Dataset)
• Add package datacite with client, models, utils and service to interact with DataCite
• Add doi field to Upload class with EmbeddedDOI document
  • Subfields: doi
• Add endpoint upload/{upload_id}/assign-doi
• Resolve DOI target url in GUI
• Testing
  • Update doi tests
  • Update dataset sets
  • Add upload test
  • Enable datacite in config to actually test in CI (affected by defaults.yaml)