When `API.ControlPlaneEndpoint` is used, the `BindPort` of the
apiserver is currently assumed, which means a load balancer cannot
listen on a different port than the apiserver. This extends the
`ControlPlaneEndpoint` to take an optional port which may differ
from the apiserver's `BindPort`.
Previously, kubeadm would use <ip>:<port> to construct a master
endpoint. This works fine for IPv4 addresses, but not for IPv6.
IPv6 requires the ip to be encased in brackets when being joined
to a port with a colon.
This patch updates kubeadm to support wrapping a v6 address with
[] to form the master endpoint url. Since this functionality is
needed in multiple areas, a dedicated util function was created.
Fixes: https://github.com/kubernetes/kubernetes/issues/48227
