add downstream testing

[yarikoptic]

So far we concentrated on establishing testing of being able to load files produced by other verisons of the hdmf/pynwb.
But it would be desired to add another category where downstream projects (e.g. dandi-cli) would be tested against that installed versions of hdmf/pynwb. Some of those might as well be "nwb extensions", but now installed/tested as regular python modules.

For this category we would get really close to what we do in https://github.com/datalad/datalad-extensions/ -- announce pypi names + github repository (+ branch), so in two separate runs we would test the latest released and current development. I guess for that we would just extend specification (#5, current version in https://github.com/dandi/nwb-healthstatus/blob/master/spec.yml) with something analogous to datalad-extensions one.