Data source objects

[TheooJ]

One of the long-term goals of skrub is to directly connect to databases. While we're not there yet, it could be interesting to start thinking about how we handle data sources and store metadata now, as some functionalities we're developing could be closely tied to a "data" object (or even belong to this object instead of the estimators). For now, it could be as simple as a dict holding tables and metadata, but I'm opening the discussion so that we can think about this