Improve memberList output consistency

[unai-ttxu]

After merging #11812, hexadecimal format for member IDs was added when json output format and --hex flag are used.

But, comparing the output of the etcdctl member list -w json and etcdctl member list -w json --hex there is one subtle difference which, I think, broke consistency.

• etcdctl member list -w json:

{
  "header": {
    "cluster_id": 8901668306595756000,
    "member_id": 13919521250792505000,
    "raft_term": 8
  },
  "members": [
    {
      "ID": 1373433005507466200,
      "name": "etcd2",
      "peerURLs": [
        "https://10.200.18.14:2380"
      ],
      "clientURLs": [
        "https://10.200.18.14:2379"
      ]
    },
    {
      "ID": 2234793613784549000,
      "name": "etcd3",
      "peerURLs": [
        "https://10.200.18.16:2380"
      ],
      "clientURLs": [
        "https://10.200.18.16:2379"
      ]
    },
    {
      "ID": 13919521250792505000,
      "name": "etcd1",
      "peerURLs": [
        "https://10.200.18.15:2380"
      ],
      "clientURLs": [
        "https://10.200.18.15:2379"
      ]
    }
  ]
}

• etcdctl member list -w json --hex:

{
  "header": {
    "cluster_id": "7b8914296afe3773",
    "member_id": "c12c12d42cfd010e",
    "raft_term": 8
  },
  "members": [
    {
      "ID": "130f6a0ab011d407",
      "name": "etcd2",
      "peerURLs": [
        "https://10.200.18.14:2380"
      ],
      "clientURLS": [
        "https://10.200.18.14:2379"
      ]
    },
    {
      "ID": "1f0394f387f08727",
      "name": "etcd3",
      "peerURLs": [
        "https://10.200.18.16:2380"
      ],
      "clientURLS": [
        "https://10.200.18.16:2379"
      ]
    },
    {
      "ID": "c12c12d42cfd010e",
      "name": "etcd1",
      "peerURLs": [
        "https://10.200.18.15:2380"
      ],
      "clientURLS": [
        "https://10.200.18.15:2379"
      ]
    }
  ]
}

As we can see, besides the format of the member ID, client URLs key changes from clientURLs to clientURLS when --hex flag is used.

This PR will set clientURLs as key value for any type of format output in order to improve consistency.

[ahrtr]

LGTM

Thank you @unai-ttxu