Multicluster - Revisit namespace selector

[nrfox]

Epic: https://github.com/kiali/kiali/issues/5620

The namespace selector allows Kiali to filter multiple pages based on the selected namespaces. It works across the graph and the list pages. In a multi-cluster deployment, namespaces are no longer unique and you can end up with duplicate entries in the namespace selector:

Some previously discussed proposals have been:

1. A separate cluster selector in addition to the namespace selector.
2. Adding cluster info to the namespace selector.
3. Keep the duplicated namespaces and just use filters on the list/graph pages themselves to filter by cluster.
4. Filter the duplicate namespaces that are shared across cluster.

4 has been chosen for now unless a better proposal emerges or other proposals are revisited.

[leandroberetta]

Another option can be to use the namespace sameness principle and just show unique entries, no matter in which clusters the namespaces are.

Example: when selecting the bookinfo, two cards will appear, one for east and other one for west.

Of course we would need to add a filter for cluster (I guess we agreed on this already also for list pages).