filter: split JsonResource's nested/flattened resource support for testing

[srl295]

• right now, impl.JsonResource does the mapping between flattened (jsonpath) and unflattened (in the JSON file) format.
• since the code requires a GP instance, it's harder to test compatibility (such as with gp-js-flatten in Javascript).

Refactor the code to improve testability.

It might be possible to do a general refactor, which would support arbitrary other filters being tested as well, or something specific to JsonResource.