Draft: add proxy provider

[h0cheung]

Use with a selector outbound. After fallback and url-test are supported, I'll add provider support for them.

Example:

{
  "type": "selector",
  "tag": "proxy",
  "providers": [
    {
      "url": "https://example.com/sub",
      "filter": "",
      "interval": 43200
    }
  ]
}

filter and interval are optional.

• [ ] Clash subscription
  • [x] Shadowsocks
  • [x] Trojan
  • [ ] other protocols
• [ ] other subscription
• [ ] cache in file

[yuumimi]

建议 subscription 支持下 sing-box 自己的格式。其它格式作为锦上添花。