Support grouping of clusters

[perotinus]

/sig multicluster

This is mainly to support authorization on groups of clusters. Namespaces are the obvious mechanism for this, but they are not the only one and we need to do some design work here to find the right approach.

From https://github.com/kubernetes/community/blob/master/contributors/design-proposals/multicluster/cluster-registry/project-design-and-plan.md#beta

[perotinus]

There is some more discussion about this in this doc.

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