Reorganize API code structure

An attempt to reorganize the API structure based on the Chi todos-resource example.

What's missing:

1. tests if both routes /apis and /apis/{id} call the correct handler. Right now we test the handlers, but not if the handlers are correctly assigned to the specific routes.
2. move the test-data-{valid,invalid} directories to another place + remove duplicates

@bartjkdp I'd love to hear your feedback and improve on the missing parts in a future MR.