Fixes#4138
When KubeSpan is enabled, Talos automatically generates or loads
KubeSpan identity which consists of Wireguard key pair. ULA address is
calculated based on ClusterID and first NIC MAC address.
Some code was borrowed from #3577.
Example:
```
$ talosctl -n 172.20.0.2 get ksi
NODE NAMESPACE TYPE ID VERSION ADDRESS PUBLICKEY
172.20.0.2 kubespan KubeSpanIdentity local 1 fd71:6e1d:86be:6302:e871:1bff:feb2:ccee/128 Oak2fBEWngBhwslBxDVgnRNHXs88OAp4kjroSX0uqUE=
```
Additional changes:
* `--with-kubespan` flag for `talosctl cluster create` for quick testing
* validate that cluster discovery (and KubeSpan) requires ClusterID and
ClusterSecret.
Signed-off-by: Andrey Smirnov <andrey.smirnov@talos-systems.com>
Signed-off-by: Seán C McCord <ulexus@gmail.com>
Co-authored-by: Seán C McCord <ulexus@gmail.com>
