Is your enhancement request related to a problem? Please describe.

This is an informational NPEP to document a best common practices for multi-cluster NetworkPolicy in a flat network model, aligning with conventions from SIG-Multicluster. The goal is to provide a reference for users and implementers. This does not propose any API changes or dictate any behaviors on implementations

Describe the solution you'd like

Multi cluster is a complex topic since it conflates multiple problems, but some users just may ONLY want to "stretch" their cluster and apply the same standard Network Policies they have in the single cluster to the objects in the "clusterset".

Describe alternatives you've considered

Some bad ideas is to embed the notion of multicluster in the API, as it makes the work unfeasible since it requires to deal with the whole multicluster and identity problem, see #308

Additional context

This is already used in production by Calico so is a behavior we know it works, just documenting it for sharing the experience with the community