Allow extending aha.auth for building new importers

Who would benefit?

Users of Aha! Develop who build custom exporters

What impact would it make?

Right now, it is unclear how to build an exporter with regards to authentication. The aha.auth API only supports authentication with services for which Aha! already has extensions. It defeats the purpose of allowing people to build custom exporters because any call to a third-party dev tool requires authentication, and it isn't clear how it can be done within the exporter.

How should it work?

Allow adding new services to aha.auth API. Provide clear documentation how it should be done. Right now, the docs say that one can build an exporter for an unsupported dev tool, but when I try to create one, I have to stop at the auth call because there's no explanation how the authentication should work.