pd-ctl, config: some configs are camel case in the result

## Enhancement Task
```
./bin/pd-ctl config show all                                     
....
  "TickInterval": "500ms",
  "ElectionInterval": "3s",
  "PreVote": true,
...
  "WarningMsgs": null,
  "DisableStrictReconfigCheck": false,
  "HeartbeatStreamBindInterval": "1m0s",
  "LeaderPriorityCheckInterval": "1m0s",
 ...
```
Should we hide them or provide JSON for them?