Support for multiple control planes in a single cluster (Maistra multi-tenancy)

One of, if not the primary differentiating feature of Maistra over Istio is its multi-tenant capabilities that allowed multiple control planes to be deployed within a single cluster such that they could operate in relative isolation. While there are other ways to implement multi-tenancy in Istio (such as with a single mesh using sidecars and traffic policies to segregate tenants, or multi-cluster), the topology of having multiple control planes within a single cluster remains an important use case to support.

Experimental work has started upstream to support a similar topology (https://istio.io/latest/docs/setup/install/multiple-controlplanes/ ) using revisions and discoverySelectors. This is the most likely place to be able to build on this capability with the Istio community, but the feature will likely take some time to mature.

This epic is to:

a) Describe / Document different approaches to multi-tenancy in Istio (Single mesh per cluster, multiple mesh per cluster and multi-cluster). This is important to document with OSSM3, even in the context of a tech preview release. jforce1 made some relevant notes here: https://gitlab.consulting.redhat.com/jforce/ossm-considerations/-/tree/master. There is also this Kubecon talk: https://www.youtube.com/watch?v=w3d8gxGpaNQ&t=128s

b) Investigate upstream support for multiple control planes per cluster and identify the work necessary to support it at the same level as upstream on OSSM3 (experimental = dev preview w/ a smoke test, alpha = tech preview, beta = ga).