Set up API contract testing

[sergei-maertens]

The NLX integration is a bit fragile because of external breaking changes.

Rework is in the pipeline to consume the directory API directly rather than through the outway, so we can set up some automated contract testing that this API returns the expected responses.

• Set up periodic test (daily?) through github actions
• Code/tests should invoke the utility functions and verify the shape of response data
• Generate certificates for the demo network and include them in repo secrets. Note that these will have to updated yearly.