Update example filters to work with arrays

[jmandel]

This should address #420 by making the examples compatible with the real-world variability of the data formats. A few notes:

• Managing this optionality in JSON Schema within filters is pretty brutal, but I do think this reflects what it takes to use the current design to accomplish even these relatively simple objectives like "find creditCard VCs that participate in the worldbankfederation.com trust scheme".

• I'm not sure whether/when tests are run in the current build process. I tried npm run test on the main branch before making changes, and this failed with "Cannot find module '../fixtures/ajv'".

• It'd be nice to have tests that run the example filters against some "known matching" and "known non-matching" example VCs, but that's probably a consideration for another PR

[bumblefudge]

opted for #423 instead, opened tracking issues to return to the holistic overhaul of examples later