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- Fix: remove outdated and duplicate servers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/constants/servers_test.go

@@ -108,7 +108,7 @@ func Test_versions(t *testing.T) {
 		"Surfshark": {
 			model:   models.SurfsharkServer{},
 			version: allServers.Surfshark.Version,
-			digest:  "58de06d8",
+			digest:  "3ccaa772",
 		},
 		"Torguard": {
 			model:   models.TorguardServer{},