chore(validation): move functions from constants

- Move validation functions from `internal/constants` to `internal/configuration/settings/validation`
- Concatenate all OpenVPN constants in `internal/constants/openvpn.go`

internal/configuration/settings/validation/surfshark.go

@@ -0,0 +1,60 @@
+package validation
+
+import (
+	"sort"
+
+	"github.com/qdm12/gluetun/internal/constants"
+	"github.com/qdm12/gluetun/internal/models"
+)
+
+func SurfsharkRegionChoices(servers []models.SurfsharkServer) (choices []string) {
+	choices = make([]string, len(servers))
+	for i := range servers {
+		choices[i] = servers[i].Region
+	}
+	return makeUnique(choices)
+}
+
+func SurfsharkCountryChoices(servers []models.SurfsharkServer) (choices []string) {
+	choices = make([]string, len(servers))
+	for i := range servers {
+		choices[i] = servers[i].Country
+	}
+	return makeUnique(choices)
+}
+
+func SurfsharkCityChoices(servers []models.SurfsharkServer) (choices []string) {
+	choices = make([]string, len(servers))
+	for i := range servers {
+		choices[i] = servers[i].City
+	}
+	return makeUnique(choices)
+}
+
+func SurfsharkHostnameChoices(servers []models.SurfsharkServer) (choices []string) {
+	choices = make([]string, len(servers))
+	for i := range servers {
+		choices[i] = servers[i].Hostname
+	}
+	return makeUnique(choices)
+}
+
+// TODO remove in v4.
+func SurfsharkRetroLocChoices() (choices []string) {
+	locationData := constants.SurfsharkLocationData()
+	choices = make([]string, 0, len(locationData))
+	seen := make(map[string]struct{}, len(locationData))
+	for _, data := range locationData {
+		if _, ok := seen[data.RetroLoc]; ok {
+			continue
+		}
+		seen[data.RetroLoc] = struct{}{}
+		choices = append(choices, data.RetroLoc)
+	}
+
+	sort.Slice(choices, func(i, j int) bool {
+		return choices[i] < choices[j]
+	})
+
+	return choices
+}