New PIA servers support (#227)

* Adapt storage: SyncServers write to file option, export FlushToFile
* CLI built-in updater for old and new PIA servers
* Update hardcoded IP addresses for PIA old and new servers
* Add PIA old to allServers struct and update timestamps
* Adapt code to work with new and old PIA servers
* Remove PIA subdomains (unneeded) from resolver tool

internal/models/server.go

@@ -1,12 +1,28 @@
 package models
 
-import "net"
+import (
+	"fmt"
+	"net"
+	"strings"
+)
+
+func stringifyIPs(ips []net.IP) string {
+	ipStrings := make([]string, len(ips))
+	for i := range ips {
+		ipStrings[i] = fmt.Sprintf("{%s}", strings.ReplaceAll(ips[i].String(), ".", ", "))
+	}
+	return strings.Join(ipStrings, ", ")
+}
 
 type PIAServer struct {
 	IPs    []net.IP `json:"ips"`
 	Region string   `json:"region"`
 }
 
+func (p *PIAServer) String() string {
+	return fmt.Sprintf("{Region: %q, IPs: []net.IP{%s}}", p.Region, stringifyIPs(p.IPs))
+}
+
 type MullvadServer struct {
 	IPs     []net.IP `json:"ips"`
 	Country string   `json:"country"`