[Feature] Add Provider bulk relay to another proxy

[kingofnull]

Verify steps

• [X] 我已经在 Issue Tracker 中找过我要提出的请求 I have searched on the issue tracker for a related feature request.
• [X] 我已经仔细看过 Documentation 并无法自行解决问题 I have read the documentation and was unable to solve the issue.

Description

I have a pc that don't have direct internet access , I had to use a socks proxy to connect to internet. So the problem is I have to define relay provider for each one and my list changes a lot. Also clash cant ping them and switch to working one.

Possible Solution

Add a option to proxy-providers and set upstream proxy name for it , then it relay all it's proxies to mentioned proxy.

example config:

proxy-providers:
  my_realyed_provider:
    type: file
    path: ./proxies.yaml
    proxy: my_interent_socks_proxy