[Feature Request] URLTest based backup group outbound selection

Hi!
Currently, the URLTest picks out an outbound by latency and tolerance from a certain group of outbounds.

I suggest a new typeof URLTest based outbound that selects the first available one from a list of outbounds.

Say it like the following structure.

{
  "type": "backup",
  "tag": "auto",
  // select the first available one from the list
  "outbounds": [
    "proxy-a",
    "proxy-b",
    "proxy-c"
  ],
  "url": "",
  "interval": "",
  "tolerance": 50, // takes no effect
  "idle_timeout": "",
  "interrupt_exist_connections": false
  // some extra fields
  //
  // switch_back_policy
  //   n(positive): switch back to the prior outbound only after it passes n times url test
  //   1: always select the first available outbound (a special case of n = 1)
  //   0: as long as the current outbound alive, keep on it
  "switch_back_policy": 0,
}

This will greatly improve the logic of outbound. Makes it more robust.
Similar implement could be found here: Fallback proxy groups

Thanks

Start by reviewing the existing URLTest outbound and the outbound configuration described in this issue, then compare the fallback proxy group behavior linked in the request. Done means a backup outbound can select the first available configured outbound, apply the stated switch-back policy, and handle the listed URL-test settings.