Implement discovery mode and stream selection

Closes #25 (closed)

I ended up running into a situation where I need discovery on this tap, so I took a stab at #25 (closed). I think there's still some room for improvement, but I wanted to lead with the minimum number of changes to start the conversation.

A few quirks:

• projects/groups get queried regardless of they are selected, because they are necessary for the other streams
• the global CATALOG is gross, but avoided adding an argument to every method
• What would be the best way to handle fetch_merge_request_commits and fetch_pipelines_extended? Ideally we'd switch these with selection. How do we deprecate them and how do they interact with selection? Perhaps pipelines_extended wouldn't need to be an extra table anymore, but that would be a much bigger breaking change