Fix column descriptors

We need to add a simple plugin that would replace `Column[X]` with `InstrumentedAttribute[Column[X], X]` and make the latter a descriptor instead instead of the former. In addition the plugin would generate a more precise `__init__`.

The plugin should act only in classes that derive from a declarative base, currently this is not always possible, so the plugin should probably also kick in for classes with fallback to `Any`.