Rewrite Outline Client's Routing for Linux

[jyyi1]

This issue is used to track the progress of rewriting the Linux routing for Outline Client.

Once the work is done, it will resolve these issues:

• Create deb package for Linux #1028
• Linux Client Permissions Issue #713
• GNU/Linux AppImage chrome-sandbox Canonical Permissions/Ownerships Issue #870
• Do not require "root" permissions (linux client) #1224
• Directory dependent shasum (/usr/bin/shasum) (Linux) #1644
• Fail to install Outline service on Linux #1690
• install_linux_service.sh failed to install outline_proxy_controller.service when /usr/local/sbin not exist #843
• Cannot find shasum #1488
• Initialization step is not working #1484
• [Bug]: Outline Client can't establish stable connection on Ubuntu 24.04 #2001

And it might resolve even more issues (for example, supporting a wider range of Linux distros).

Here is the progress:

• Convert .AppImage to .deb package (feat(client/linux): generate Debian package #2255)
• Routing Logic PoC (feat(client/linux): revamp the Linux VPN routing logic #2291)
• TCP Traffic Routing (feat(client/linux): revamp the Linux VPN routing logic #2291)
• UDP Traffic Routing (feat(client/linux): revamp the Linux VPN routing logic #2291)
• Handling concurrency (feat(client/linux): revamp the Linux VPN routing logic #2291)
• IPv4 Traffic Routing (feat(client/linux): revamp the Linux VPN routing logic #2291)
• Initial Connectivity Check (feat(client/linux): revamp the Linux VPN routing logic #2291)
• IPv6 Traffic Routing
• Refresh UDP connectivity
• Network Change Monitor
• Network Change Connectivity Check
• Delete old routing service code
• Handle cancellation gracefully