Subscription service

[qjebbs]

The PR add subscription service support. Please see document files in the PR for detail.

Together with #163, #164, we can achieve unattended running of sing-box without external tools required.

{
  "services": [
    {
      "type": "subscription",
      "tag": "all.sub",
      "interval": "1h",
      "providers": [
        {
          "tag": "provider1",
          "url": "https://url.to/provider1",
          "exclude": "",
          "include": ""
        }
      ]
    }
  ]
}