[FEATURE REQUEST] Break out a "sage-core" library

[iuliaturc]

Is your feature request related to a problem? Please describe.
We are now accumulating a lot of external dependencies. For instance, I'm now trying to add Aider and it's virtually impossible to do it because of conflicting dependencies.

Describe the solution you'd like
We should pick a main workflow (a single retriever + a single LLM) and bundle that as a core library. For any extensions, we should ask users to manually install the desired dependency. Optional dependencies via pyproject.toml sounds like a good solution.