support CITATION.cff

[yarikoptic]

https://github.com/citation-file-format/citation-file-format#:~:text=The%20Citation%20File%20Format%20(CFF,%2Dformat.github.io.

recently added to datalad

[yarikoptic]

it got widely adopted, see e.g. https://github.com/search?q=CITATION.cff&type=code . Needed to check how it interoperates with .zenodo.json, and it seems that there is some citation-file-format/citation-file-format#374 so may be there is no need to duplicate information in both?

[yarikoptic]

apparently zenodo supports it now. https://help.zenodo.org/ says

How does a CITATION.cff file affect the metadata of my GitHub release?

When we find a CITATION.cff file in your GitHub repository's root folder, we make a best-effort attempt at parsing Zenodo-compatible metadata from it.

For backwards-compatibility, if you have a .zenodo.json file in the repository, only the .zenodo.json's metadata will be taken into account. Of course, any existing valid CITATION.cff file will still be used to display the citation box on the GitHub repository page, but will not affect in any way the metadata we extract for your published release Zenodo record.

As there is an increasing use of CITATION.cff -- it would be great to add support for it.