Multi cluster / aggregated view

[illume]

Headlamp already supports multiple clusters. Users with multiple clusters, especially those with dozens of them, would find it useful to somehow group them together. This means having the notion of cluster groups, which the user can create and edit by adding/removing clusters to them, etc.
In the UI, when viewing a group it will be very similar to viewing a cluster, except we will see the resources from all the clusters in the group together. There will be also a new overview for a group, which will show useful info from all the clusters aggregated.

This is the umbrella issue so we manage this feature:

• Investigate the websocket pool max limit  #1373
• Centralize the calls for listing/getting resources from the k8s server #1700
• WIP: Allow to create/update/delete cluster groups #1701
• WIP: Add new group overview #1702
• WIP: Add new home view with groups #1703
• Allow to use the cluster argument in useList/Get functions #1747
• WIP: frontend: BindingList: Make multi cluster aware #2438
• WIP: frontend: RoleList: Make multi cluster aware #2437
• frontend: ResourceTable: Add display of clusterErrors #2436
• WIP: frontend: PodList: Make multi cluster aware #2435
• frontend: Add cluster column to List views #2434
• frontend: Fix deps with npm audit fix #2433
• frontend: ResourceTable: Add cluster column #2432
• frontend: Resource/CreateButton: Add cluster selector #2431
• frontend: EditorDialog: Add optional actions property #2428
• frontend: RouteSwitcher: Fix multicluster mode, no token check #2426
• frontend: ResourceLink: Add cluster param automatically #2425
• frontend: Add ClusterGroupErrorMessage for displaying clusters with errors #2424
• frontend: util: Add combineClusterListErrors #2423
• frontend: util: Add flattenClusterListItems for cluster objects #2422
• frontend: cluster: Add cluster argument to KubeObject class #2421
• frontend: k8s/cluster: Add cluster arg to onError callbacks #2420
• frontend: k8s/cluster: Add clusters field to ApiListOptions #2419
• frontend: lib/util: Add getClusterGroup #2416

[illume]

• There's a comment in this PR for Resource/CreateButton of something else to do.

As it's more discoverable for the user? Also, for now we may keep it single cluster selection, but it's be pretty cool to be able to select either the full cluster group or multiple individual clusters, so we could batch applies. But this can be added later.