[Feature] support auto interface-name detection for proxies when dialing over vpn

[wenerme]

验证步骤

• 我已经阅读了 文档，确认了该功能没有实现
• 我已在 Issue Tracker 中寻找过我要提出的功能请求，并且没有找到
• 我是中文用户，而非其他语言用户

描述

I have multi interface, the proxy use private ip like 10.1.1.1, I have an tun interface 10.1.1.0/24, the interface name is dynamic (macOS can not set tun name), when I use 10.1.1.1 for mihomo proxy upsteam, mihomo will use dialer.DefaultInterface, this is the default interface like en0, but this interface can not handle 10.1.1.1.

I hope maybe there is a preprocess for proxies, which use NetworkUpdateMonitor to choose the right interface-name. e.g.

# this is the work around for onetime config update
.interface-name: &interface-name utun6
.server: &server 10.1.1.1

proxies:
- name: socks
  server: *server
  # interface-name: *interface-name
  interface-name: auto
  port: 1080
  <<: *socks

when interface-name: auto, the preprocessor will replace the interface name which match server, e.g. utun6 in this case.