serializers: add datapackage serializer

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Description

Data Package is a European Commission-funded project (via NLnet) that is a standard containing a set of lightweight specifications for describing datasets and individual data files. The standard is widely used for data interoperability in other data portal systems like CKAN, Dryad, Our World in Data, and others. The main benefit of adoption the Data Package Standard is a rich set of software for reading and validating datasets - https://datapackage.org/standard/software/ (the most advanced implementations are in Python and R, as well as a Desktop application).

By creating this pull request, we would like to bootstrap a discussion if Invenio RDM (and Zenodo) would consider accepting the datapackage.json format to be one of the export targets for datasets' metadata (we also prepared a PR for invenio-app-rdm in case this PR is accepted). Also, tagging our friends from the University of California @sdiggs and @sapetti9 from Open Knowledge Foundation.

Thanks a lot in advance!

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