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- Fix: remove outdated and duplicate servers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
+}