Need documentation on cluster migration

[oaktowner]

FEATURE REQUEST

Describe the feature:

Some customers do not upgrade kubernetes clusters in place -- they spin up a new cluster with the new version of kubernetes, then deploy workloads over there, then turn down original cluster.

If they are replacing the cluster that is running the Istio control plane, they will need to ensure that they can migrate the Istio functionality safely from the original cluster to the new cluster.

Note this must work even when there are multiple clusters (possibly across multiple trust domains).

[oaktowner]

Note: this may be taken on in this existing design doc as an alternate flow.