PD-ctl should show the timestamps in a consistent way

When we call `pd-ctl store` , it shows the timestamps in 4 different ways, see below example:
```
{
"store": {
...
"git_hash": "6c1424706f3d5885faa668233f34c9f178302f36",
"start_timestamp": 1647365006, <========= 10 digits unix timestamp
...
"last_heartbeat": 1647373317633963588, <========= 19 digits unix timestamp
"state_name": "Up"
},
"status": {
....
"start_ts": "2022-03-15T17:23:26Z", <========= YYYY-MM-DDTHH:mm:ss
"last_heartbeat_ts": "2022-03-15T19:41:57.633963588Z", <========= YYYY-MM-DDTHH:mm:ss:nnnnnnnnn
...
}
}
```

It's pretty hard to evaluate the difference between unix timestamps and the date time format. Please \ show the timestamps in a consistent way.