Add the public VPN server lists, with is_vpn and vpn_providers (closes #235)

The IP lookup now checks Mullvad, NordVPN, Private Internet Access, IVPN and X4BNet's community list, each downloaded once a day. Each result has `flags.is_vpn` and `vpn_providers`.

Judgement call: X4BNet publishes IPv4 and IPv6 as two files, so they are two lists, `x4bnet` and `x4bnet_v6`.
Judgement call: Private Internet Access's signature is not read or checked.
Rule suppressed: `tagliatelle` on the structs holding the publishers' field names.

Model: opus-5-5

internal/iplist/iplist.go

@@ -34,6 +34,23 @@ const (
 	// addresses in two fields: exit_addresses, which has IPv4 only, and
 	// or_addresses, which has IPv4 and IPv6, each with a port.
 	FormatOnionooJSON
+
+	// FormatMullvadJSON is Mullvad's JSON list of relays. A relay's addresses
+	// are in ipv4_addr_in, ipv6_addr_in and ipv4_v2ray.
+	FormatMullvadJSON
+
+	// FormatNordVPNJSON is NordVPN's JSON list of servers. A server's
+	// addresses are in station and ipv6_station.
+	FormatNordVPNJSON
+
+	// FormatPIAJSON is Private Internet Access's JSON server list, then a
+	// blank line and a signature. Each region's servers has lists of servers
+	// by protocol, and each server has its address in ip.
+	FormatPIAJSON
+
+	// FormatIVPNJSON is IVPN's JSON server list. The hosts of its wireguard
+	// and openvpn gateways have their addresses in host and v2ray.
+	FormatIVPNJSON
 )
 
 // List is one public IP list.
@@ -53,6 +70,9 @@ type List struct {
 
 	// Format is how the list's file is written.
 	Format Format
+
+	// VPN says that the list holds servers of VPN services.
+	VPN bool
 }
 
 // Listing says that a list contains an address.
@@ -63,6 +83,10 @@ type Listing struct {
 	// Updated is when routewatch last downloaded the list, or last heard from
 	// its publisher that its copy was still current.
 	Updated time.Time `json:"updated"`
+
+	// VPN says that the list holds servers of VPN services. A lookup answer
+	// reports it through is_vpn and vpn_providers.
+	VPN bool `json:"-"`
 }
 
 // listCopy is a list's last good copy in memory.
@@ -129,6 +153,7 @@ func (b *IPLists) Lookup(addr netip.Addr) []Listing {
 				Name:        list.Name,
 				Description: list.Description,
 				Updated:     c.updated,
+				VPN:         list.VPN,
 			})
 		}
 	}