Similar to how we support implicit instance attributes, we should also support implicitly defined class attributes. More concretely, we want the TODO comment here to be resolved, i.e. there should be no unresolved-attribute diagnostic:

https://github.com/astral-sh/ruff/blob/eb08345fd5434ed3db243233df6dd91757a6af3b/crates/red_knot_python_semantic/resources/mdtest/attributes.md?plain=1#L503-L514

The implementation for this can probably re-use most of the machinery introduced in this PR. In particular, the Type::implicit_instance_attribute method.