Add device discovery and custom-devices

[HidenoriMatsubayashi]

I added device discovery for remote devices and customized-custom-devices. We need to create ~/.flutter_custom_devices.json file to handle remote devices.

e.g.

{
  "custom-devices": [
    {
      "id": "embedded-linux",
      "label": "Embedded Linux Device",
      "sdkNameAndVersion": "Embedded Remote device",
      "enabled": true,
      "platform": "arm64",
      "backend": "wayland",
      "ping": [
        "ping",
        "-w",
        "500",
        "-c",
        "1",
        "localhost"
      ],
      "pingSuccessRegex": "ttl=",
     }
  ]
}

Related issues

• Discovery remote devices #12