text: Title: Add support for DRYAD data repository
  Body: [Sorry if my issues sound like a broken record! I hope more contentproviders will open up BinderHub for more communities.]
  
  ### Proposed change
  
  Support DRYAD (or [stash](https://github.com/CDL-Dryad/stash)) as a content provider (DOI + URL-based): https://datadryad.org/
  
  Their guide to data sharing explicitly mentions code + data, see https://datadryad.org/docs/QuickstartGuideToDataSharing.pdf, so _they should be a good fit_.
  
  _The biggest problem right now_ is that I did not yet find a suitable repository (chicken and egg problem?!), and DRYAD is [not for free](https://datadryad.org/stash/publishing_charges) unless you are affiliated with specific institutions.
  
  **Anyone here who can upload a test binder to DRYAD?**
  
  Relevant API documentation: 
  
  - https://datadryad.org/api/v2/docs/#/default/get_datasets__doi_
  - https://datadryad.org/api/v2/docs/#/default/get_datasets__doi__download (zipped and direct file download possible, didn't see any limitation on ZIP archive size)
  
  ### Alternative options
  
  None? :fist_raised:
  
  ### Who would use this feature?
  
  DRYAD is widely known in some research communities, e.g. ecology, so any researcher publishing a dataset or workflow there would be familiar with it.
  
  ### How much effort will adding it take?
  
  1 day, with the current examples of contentproviders as a boilerplate.
  
  ### Who can do this work?
  
  A developer familiar with the content provider implementation.
  