[Feature] Fleet: Use member cluster labels to group clusters for update strategies

As a platform operator managing a large Kubernetes fleet, I want to define stages and groups in update strategies using **member cluster labels** instead of a member cluster's `updategroup` property so that I have more flexibility in how I place clusters in different strategies.

As an example, I do an early-stage control plane upgrades by having a label of "k8scpupdate:1" and late-stage node image updates for the same cluster by having a second label of "nodeimgupdate:3". I can then use the two labels in different update strategies providing increased flexibility.