Add "IPv6 interface" setting to Advanced settings

[ameshkov]

Issue Details

The user may want to control the state of IPv6 for the VPN network interface and we currently do not provide this capability.

Proposed solution

I suggest doing it in the same way as its done in iOS/Android, i.e. provide a setting "IPv6 interface" similar to what iOS/Android provide: AdguardTeam/AdGuardVPNForiOS#410

It should have three states:

• Auto -- add IPv6 interface if the current network interface has it.
• Enabled -- forcibly add IPv6 address to Tun interface.
• Disabled -- never add IPv6 address to Tun interface.

Alternative solution

No response