Change the way "IPv6 interface" setting works

[ameshkov]

Issue Details

Currently, it is basically has two states:

• Auto -- when the setting is enabled, it is actually not enforced, but only added when the underlying network has IPv6 access.
• Disabled -- do not add IPv6 interface to Tun at all.

The problem is that there're use cases when the user may want to get IPv6 access with the help of AdGuard VPN.

Proposed solution

I suggest changing it to three states:

• Auto -- the same as when it's enabled now
• Enabled -- forcibly add IPv6 address to Tun interface.
• Disabled -- never add IPv6 address to Tun interface.

Alternative solution

No response