Enable Prow plugins for etcd PRs

### What would you like to be added?

Plugin enablement for etcd in [test-infra](https://github.com/kubernetes/test-infra/blob/master/config/prow/plugins.yaml).
Suggested plugins to be added:
- [x] cherrypicker: https://prow.k8s.io/command-help#cherrypick
- [x] size: https://github.com/kubernetes/test-infra/blob/master/config/prow/plugins.yaml#L145-L151
- [x] needs-rebase: https://github.com/kubernetes/test-infra/blob/master/config/prow/plugins.yaml#L1661-L1664
- [x] lgtm: https://prow.k8s.io/command-help#lgtm
- [x] approve: https://prow.k8s.io/command-help#approve
- [x] hold https://prow.k8s.io/command-help#hold
- [x] lifecycle https://prow.k8s.io/command-help#lifecycle

### Why is this needed?

There are useful Prow plugins that etcd can leverage to minimize manual efforts. One of them is the cherrypicker plugin, which can backport any PR to a specific branch with just a `/cherry-pick ` comment on the PR.
Example: https://github.com/containerd/containerd/pull/10257#issuecomment-2132261738