PATCH: add support for printing keymgr list in JSON

The attached patch adds support for printing keymgr zone list in JSON as suggested earlier today.

JSON is triggered by option -j (JSON). I have also used the json.[ch] which have been used in the existing kdig and stats MR.

The list of keys is always a JSON array:

$ keymgr one.example list -j
[
  {
    "zone": "one.example.",
    "id": "0735cbe3ae4e76e671037b079d004265edb89c09",
    "ksk": true,
    "zsk": false,
    "tag": 16106,
    "algo": 13,
    "size": 256,
    "public-only": false,
    "pre-active": 0,
    "publish": 1652185541,
    "ready": 1652185541,
    "active": 1652185541,
    "retire-active": 0,
    "retire": 0,
    "post-active": 0,
    "revoke": 0,
    "remove": 0
  }
]

Timestamps are integer if UNIX epoch format was requested, string otherwise:

"publish": "2022-05-10T12:25:58Z",
...
"publish": "-1h7m24s",
...
"publish": 1652185558,

0001-add-support-for-printing-keymgr-list-in-JSON.patch