Add ro-crate support for exporting (and importing?) Data Resources

[github-actions]

Original issue description

Is your feature request related to a problem? Please describe.
Currently, base-repo allows to download data, optionally in a zip file. Metadata can also be accessed via RESTful endpoints, but is kept separated from downloaded data. There is no way to download a self-describing package of entire Data Resources.

Describe the solution you'd like
Providing support for RO-Crates could tackle this issue. Allowing to access Data Resources with a special content-type, which produces an RO-Crate for the entire Data Resource, might be helpful, e.g., for archiving contents. Optionally, RO Crates could also be used for importing Data Resources, e.g., for migration.

closes #170

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