feat: Enable sharding of ResourceGroup objects

[rquitales]

This commit enables sharding of ResourceGroup objects to scale the number of GKNN resources tracked within an inventory. Prior to sharding, the number of objects tracked is limited to the size of the ResourceGroup object and is bottlenecked by etcd size limits. This modification allows us to distribute GKNNs to multiple 'paginated' ResourceGroups.

[karlkfi]

/hold

Per offline discussion, the cli-utils ClusterClient needs refactoring to push more of the inventory lifecycle management into the provider/storage layer.