Introducing multicluster support for ambient mode (alpha)

Multicluster has been one of the most requested features of ambient -— and as of Istio 1.27, it is available in alpha status! We sought to capture the benefits and avoid the complications of multicluster architectures while using the same modular design that ambient users love. This release brings the core functionality of a multicluster mesh and lays the groundwork for a richer feature set in upcoming releases.