Add code, CLI for interacting with TDC CKAN

[khaeru]

Now that the TDC CKAN instances (production and development) are running, this adds PR code and command-line tools for interacting with these via the Client class created in #35.

Also:

• Add a test mark to prevent running tests against either CKAN instance during GitHub Actions CI workflows.
• Add a CLI group for testing utilities, tdc test.
  • Add a command tdc test wipe to remove all CKAN packages from the "TDCI Test Organization" used by the test suite.

How to review

Try some or all of the following:

1. In a terminal, run: tdc ckan tag_list.
2. Show the help for a given command: tdc ckan help_show name=tag_list.
3. Run a command with some arguments: tdc ckan tag_list all_fields=true.
4. Run against the development instance: tdc ckan --instance=dev tag_list all_fields=true.
5. Add an API token to your keyring, then make a query that requires authentication:

   keyring set transport-data api-token-prod
   keyring set transport-data api-token-dev
   tdc ckan user_list
   

Also:

• Read the added documentation.
• Run the test suite.

PR checklist

• Checks all ✅
• Update documentation
• Update doc/whatsnew.rst