[red-knot] Support implicit class attributes (only defined in a @classmethod)

[sharkdp]

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:

ruff/crates/red_knot_python_semantic/resources/mdtest/attributes.md

Lines 503 to 514 in eb08345

	class C:
	@classmethod
	def class_method(cls):
	cls.pure_class_variable = "value set in class method"
	# for a more realistic example, let's actually call the method
	C.class_method()
	# TODO: We currently plan to support this and show no error here.
	# mypy shows an error here, pyright does not.
	# error: [unresolved-attribute]
	reveal_type(C.pure_class_variable) # revealed: Unknown

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