Previously we would use the Impersonate-Group header to pass through
tags to the k8s api server. However, we would do nothing for non-tagged
nodes. Now that we have a way to specify these via peerCaps respect those
and send down groups for non-tagged nodes as well.
For tagged nodes, it defaults to sending down the tags as groups to retain
legacy behavior if there are no caps set. Otherwise, the tags are omitted.
Updates #5055
Signed-off-by: Maisem Ali <maisem@tailscale.com>
