Need to check if adding fields to API responses would be considered
a breaking change. Assume it isn't, for now.

These were accidentally commented out when debugging

Add tests that consume every object on every endpoint, and
make adjustments where this has uncovered incorrect types.

Using -1 might be a bug. It only works on the public API,
not when self-hosted. Use 9999 for now as it works on both and
wait for feedback about -1.

Bonus: don't create mockito objects if it is not being used

Now the Id trait is implemented for (Named)APIResource despite
the lack of an id field!

(Named)APIResource objects now have a .get() method which retrives
the resource they point to

Now we know what type the json they link to describes

Including tests

The endpoint can now be set at runtime. This allows the tests to
use HTTP mocking and allows compilation on stable.

Can retrieve Berries from the API. Responses are cached.