Create a custom cache index for Cluster.Topology.Class

[fabriziopandini]

Detailed Description:
This is part of the activities for the implementation of the ClusterClass proposal.

In order to setup a watch that triggers reconciliation for all the Clusters using a ClusterClass object being update, it is required to create new cache index on the Cluster.Topology.Class.

Such index will be used in the map function from ClusterClass to Clusters.

Anything else you would like to add:
This could be implemented only after #4908 is completed.

See eg. https://github.com/kubernetes-sigs/cluster-api/blob/master/controllers/noderefutil/indexer.go for the index & map function between machines & nodes

/kind feature

[ykakarap]

/assign