Add an offline etcdutl command to cleanup any custom content in v2store

### What would you like to be added?

We already provided a `etcdutl check v2store` command to check whether the v2store contains any custom content. We should also provide a command to cleanup any custom content if any.

See discussion in https://github.com/etcd-io/etcd/discussions/20231

### Why is this needed?

To remove blocker of upgrading from 3.5 to 3.6.