Migrate Kubernetes extension commands

[msivasubramaniaan]

Replace extension.vsKubernetesLoad, extension.vsKubernetesUseNamespace command functionality

Ref: #3981

[vrubezhny]

It doesn't look like we really need anything to do with extension.vsKubernetesUseNamespace - we use openshift.project.set to set/change active namespace/project from Command Palette/Status Bar/Context menu.
I believe we can just remove any extension.vsKubernetesUseNamespace registrations/references as not used.

[vrubezhny]

For the extension.vsKubernetesLoad command we already have a replacement: openshift.resource.load. The only problem here is that when we remove the k8s extension dependency then we get rid of KubernetesResourceVirtualFileSystemProvider virtual file system provider that, as far as I understand, is registered by the k8s extension to respond to URI requests to load deployment/pod/etc. yaml-files like k8smsx(ro)?://loadkubernetescore/, k8smsx(ro)?://kubernetesdescribe/, k8smsx(ro)?://helmget/ - so we have to add this virtual file system provider to vscode openshift tools in order to make it working correctly without having the k8s extension installed.
However some other modifications/fixes might be also required, f.i., Implement Port Forward IU (add/remove/show related information) #3906