[FR] json output for `proxy list`

Thanks for this awesome project! I want to use `clashctl` in some of my scripts, but parsing the output of `proxy list` is a pain.

Maybe we can add a json output option for easy parsing. e.g. with a schema like:
```json
{
  "name": "proxygroup1",
  "type": "Selector",
  "delay": null,
  "subproxies": [
    "proxy1",
    "proxy2",
    "proxygroup2"
  ]
}
```