handle `message` output in Zenodo and bibtex backends

[alexlancaster]

I notice that the message key in CITATION.cff isn't being translated to anything on the Zenodo side.

The equivalent on Zenodo appears to be an additional_description with the type of notes, which formats it in a box to tell the consumer how to use the citation, e.g. https://zenodo.org/records/10648000

This PR implements this translation, turning:

- message: If you use this software, please cite it using these metadata.

into:

"additional_descriptions": {
    "description": "If you use this software, please cite it using these metadata.",
    "type": {
      "id": "notes"
    }
  },

This would again, be another step towards needing to maintain this message in two places.

[alexlancaster]

added support for bibtex output as per discussion on #158 .

[alexlancaster]

To summarize:

• Zenodo: message -> additional_descriptions
• BibTeX: message -> note