gh: Use toml for new hub's config

-[x] Introduce toml for config
-[x] Fix cukes setup steps to use toml
-[ ] Fix `authentication.feature`
-[ ] Backward compatibility with the old hub's yaml format?

@mislv I'm not sure how much backward compatibility we need to support
old hub's yaml format. But I think we have some flexibility there.
We could just let our users know they need to reauthenticate for the new hub.

Changes are still in progress and I created this PR to provoke discussion before I go further