KCP does not update api-server's list of etcd node ips

KCP does not update api-server's list of external etcd's node ips when the etcd node's ip change from DHCP issues. This happens especially when just the etcd node's ip changes without the node going down.

Start by tracing how KCP handles external etcd membership and how the api-server receives its etcd node IP list when DHCP changes an etcd node IP without a node shutdown. Confirm the expected update path and add regression coverage for this scenario; done when the api-server uses the new IP without requiring the etcd node to go down.

Written by the indexing model from the issue text.