Provide explicit definition of variables scope in ClusterClass

User Story

As a user, I would like to understand what variables are meant to be used as overrides at CP/MD levels

Detailed Description

As of today, ClusterClass allows to define variables and all those variables can be used in a Cluster's topology.variables as well as for providing override at CP and MD level.

However, in practice, it really makes sense to use variables as override only a subset of variables, the ones that are used by patches impacting CP/MD of a given class.

This is currently hard to detect, especially for users which are not the ClusterClass authors, so it will be great if we can extend the current API by providing explicit information about the scope where a variable can be used.

Anything else you would like to add:

Might be it is possible to Inferr variables scope from patches, but it doesn't seem the right way to go because it will be complex and brittle.

/kind feature