Display full list of DNS zones in VNet panel (#43195)

* Remove unused getCluster func from clusterConfigCache

* Make ClusterConfigCache public

* Improve warning when fetching leaf clusters for DNS setup

* Return AlreadyExists from VnetService.Start and handle it on frontend

* Implement VnetService.ListDNSZones

* Share ClusterConfigCache between lib/vnet and lib/teleterm

* Extract blink keyframes

* Add processing and warning status to ConnectionStatusIndicator

* Speed up slide animation for VNet panel

* Display DNS zones in VNet panel

* Show processing state on repeated calls only after delay

* Pass empty deps array to useEffect

* Return Promise<never> from usePromiseRejectedOnUnmount

* Add ellipsis to processing state copy

* MockedUnaryCall.then: Handle onrejected not being defined

I just copied stuff from the linked implementation of then.

* Fix mistake in comment for ListDNSZones