What would you like to be added?

Part of #12913

etcd 3.6 will panic if the v2store contains custom content. Before users upgrade from 3.5 to 3.6, they must ensure there is no any custom content in v2store; and both the online and offline commands need to check it. For the offline command, it also needs to check the WAL records after the latest v2 snapshot file.

The proposed commands:

etcdctl check v2store
etcdutl check v2store

EDIT: We only need to add the offline tool. For online tool, Users should be able to use the existing etcdctl v2 command ETCDCTL_API=2 ./bin/etcdctl ls directly, refer to #19114 (comment)

Also --enable-v2 and --experimental-enable-v2v3 have already been removed i 3.6, so users must ensure that they do not configure these two flags before upgrading to 3.6. Only the online command checks it.

Why is this needed?

To ensure smooth upgrade from 3.5 to 3.6.