Refactor tunnel state machine DNS configuration

[dlon]

This PR makes the tunnel state machine DNS configuration more flexible:

enum DnsConfig {
    /// Use tunnel config gateway addresses
    Default,
    /// Use the specified addresses for DNS resolution
    Override {
        /// Addresses to configure on the tunnel interface
        tunnel_config: Vec<IpAddr>,
        /// Addresses to allow on non-tunnel interfaces
        non_tunnel_config: Vec<IpAddr>,
    },
}

Previously, the configuration consisted simply of a list of IPs (Option<Vec<IpAddr>>), and whether an IP was configured for the tunnel device or not was decided based on whether IPs were private/public.

Motivation: This makes it possible to ensure that certain private IPs, like IPv6 content blockers, are only allowed on the tunnel interface, without making the talpid universe aware of these IP ranges.

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This change is 

[MarkusPettersson98]

Reviewed 23 of 23 files at r1, 11 of 11 files at r2, all commit messages.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @dlon)

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mullvad-daemon/src/dns.rs line 17 at r1 (raw file):

/// Return the resolvers as a vector of `IpAddr`s. Returns `None` when no special resolvers
/// are requested and the tunnel default gateway should be used.

This comment is outdated

Code quote:

/// Return the resolvers as a vector of `IpAddr`s. Returns `None` when no special resolvers
/// are requested and the tunnel default gateway should be used.

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mullvad-daemon/src/dns.rs line 108 at r1 (raw file):

    }

    // Public IPs should be tunneled, private IPs should not be, except gateway?

Question mark?

Code quote:

// Public IPs should be tunneled, private IPs should not be, except gateway?

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talpid-core/src/dns/mod.rs line 100 at r1 (raw file):

        v.extend(self.non_tunnel_config);
        v
    }

Nit This could easily be generalized and hide inner representation of how addresses are stored in ResolvedDnsConfig

    /// Consume `self` and return an iterator of all addresses
    pub fn addresses(self) -> impl Iterator<Item = IpAddr> {
        let mut v = self.tunnel_config;
        v.extend(self.non_tunnel_config);
        v.into_iter()
    }

Code quote:

    /// Consume `self` and return a vector of all addresses
    pub fn addresses(self) -> Vec<IpAddr> {
        let mut v = self.tunnel_config;
        v.extend(self.non_tunnel_config);
        v
    }

[dlon]

Reviewable status: 19 of 23 files reviewed, 1 unresolved discussion (waiting on @MarkusPettersson98)

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mullvad-daemon/src/dns.rs line 17 at r1 (raw file):

Previously, MarkusPettersson98 (Markus Pettersson) wrote…

This comment is outdated

Done.

[MarkusPettersson98]

Reviewed 4 of 4 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved

[MarkusPettersson98]

Reviewed 2 of 2 files at r4, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved