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/helpers.go

@@ -0,0 +1,23 @@
+package validation
+
+import "sort"
+
+func makeUnique(choices []string) (uniqueChoices []string) {
+	seen := make(map[string]struct{}, len(choices))
+	uniqueChoices = make([]string, 0, len(uniqueChoices))
+
+	for _, choice := range choices {
+		if _, ok := seen[choice]; ok {
+			continue
+		}
+		seen[choice] = struct{}{}
+
+		uniqueChoices = append(uniqueChoices, choice)
+	}
+
+	sort.Slice(uniqueChoices, func(i, j int) bool {
+		return uniqueChoices[i] < uniqueChoices[j]
+	})
+
+	return uniqueChoices
+}