Change the way "IPv6 interface" setting works

### 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_