export formats: add swhid to datacite, bibtex, codemeta, cff (don't trigger metadata-only deposit when only swhid was added)

[alejandromumo]

Add swhid to serializers, without adding a field in the metadata of the record (i.e. no record updates)

Unresolved questions:

• Maybe add a system field, but the module is optional
• Do we make a dumper and make them configurable?
• first DOI registration won't have this but then has to be updated

[santolucito]

will this let us generate bibtex citations for artifacts through the web interface?