mullvad-daemon: add option to only manage the TUN interface and not touch the system

[learnwayback]

Feature description

With the introduction of DAITA, merely using wireguard w/ mullvad is no longer an option if one wishes to make use of DAITA.

The workflow of setting up a wireguard interface in netns1 and then moving the wg interface to netns2 does not appear possible with mullvad-daemon (or if it is, the daemon will likely corrupt netns1 and generate errors at the inconsistent state).

Alternative solutions

Manual use of GotaTun was considered but apparently the DAITA integration runs through mullvad-daemon. not viable?

Type of feature

• Better privacy/anonymity
• Better at circumventing censorship
• Easier to use
• Other

Operating System

• Android
• iOS
• Windows
• macOS
• Linux