Feat: rework Surfshark servers data (#575)

- Feat: `MULTIHOP_ONLY` variable
- Feat: `COUNTRY` variable
- Feat: `CITY` variable
- Feat: `REGION` variable, with retro-compatibility
- Feat: merge servers from API, zip and hardcoded hostnames
- Fix: remove outdated and duplicate servers
- Maint: faster update with fully parallel DNS resolutions

internal/models/location.go

@@ -0,0 +1,12 @@
+package models
+
+// SurfsharkLocationData is required to keep location data on Surfshark
+// servers that are not obtained through their API.
+type SurfsharkLocationData struct {
+	Region   string
+	Country  string
+	City     string
+	RetroLoc string // TODO remove in v4
+	Hostname string
+	MultiHop bool
+}

internal/models/server.go

@@ -119,7 +119,11 @@ type PurevpnServer struct {
 
 type SurfsharkServer struct {
 	Region   string   `json:"region"`
+	Country  string   `json:"country"` // Country is also used for multi-hop
+	City     string   `json:"city"`
+	RetroLoc string   `json:"retroloc"` // TODO remove in v4
 	Hostname string   `json:"hostname"`
+	MultiHop bool     `json:"multihop"`
 	TCP      bool     `json:"tcp"`
 	UDP      bool     `json:"udp"`
 	IPs      []net.IP `json:"ips"`