[3.5] *: support custom content check online in v2store

[fuweid]

It's to use v2 API to list all keys in v2store. If there is any active key in v2store, etcdctl check v2store returns error. However, the last snapshot might still contain custom content. The end-user still needs to check that content offline by etcdutl check v2store.

Part of #18993

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

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[ahrtr]

Users should be able to use the existing etcdctl v2 command ETCDCTL_API=2 ./bin/etcdctl ls directly,

etcd/etcdctl/ctlv2/ctl.go

Line 63 in 762e938

command.NewLsCommand(),

Firstly start etcd with v2 enabled,

$ ./bin/etcd  --enable-v2

Then run commands below,

$ ETCDCTL_API=2 ./bin/etcdctl ls
//Nothing returned

$ ETCDCTL_API=2 ./bin/etcdctl set k1 v1
v1

$ ETCDCTL_API=2 ./bin/etcdctl ls
/k1

$ ETCDCTL_API=2 ./bin/etcdctl set k2 v2
v2

$ ETCDCTL_API=2 ./bin/etcdctl ls
/k2
/k1

If v2 isn't enabled at all,

$ ETCDCTL_API=2 ./bin/etcdctl ls
Error:  client: response is invalid json. The endpoint is probably not valid etcd cluster endpoint

Similarly, users can run ETCDCTL_API=2 ./bin/etcdctl user list and ETCDCTL_API=2 ./bin/etcdctl role list to get the user and role list,

$ ETCDCTL_API=2 ./bin/etcdctl user add  u1:p1
User u1 created

$ ETCDCTL_API=2 ./bin/etcdctl user add  u2:p2
User u2 created

$ ETCDCTL_API=2 ./bin/etcdctl user add  u3:p3
User u3 created

$ ETCDCTL_API=2 ./bin/etcdctl user list
u1
u2
u3

$ ETCDCTL_API=2 ./bin/etcdctl role add r1
Role r1 created

$ ETCDCTL_API=2 ./bin/etcdctl role list
r1
root

[fuweid]

Users should be able to use the existing etcdctl v2 command ETCDCTL_API=2 ./bin/etcdctl ls directly,

Yes. The v2 api is that only way to check v2store online. Let me close it. Thanks for the comment @ahrtr