Support external etcd

[vaizki]

Feature Request

Allow using any etcd cluster in Talos control plane nodes and add a configurable etcd prefix to namespace different clusters in one etcd.

Description

Now that Talos CP can be run in Pods it would often make sense to use a separate external etcd in cases where you want to run many Talos clusters with the control plane hosted inside a k8s cluster:

• Pod IPs are ephemeral and cannot be made static in k8s leading to a complete loss of etcd cluster if all members change their IPs, for example in a case where volumes can't be mounted, too many k8s nodes are rebooted or lost etc. The etcd cluster has no official mechanism to recover the cluster after this.
• Most clusters can run just fine with one CP but having just one etcd member is iffy
• It would be much easier to scale CPs up & down if the etcd was not included
• Talos Pods could run off k8s local storage like hostPath with more confidence as the etcd is safe elsewhere
• Backups & maintenance could be centralized
• Performant disks (NVMe) can be more easily allocated to etcd
• etcd operator for k8s already solves all the unique problems of etcd-inside-k8s, including the all-members-change-IP problem