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/windscribe.go

@@ -0,0 +1,27 @@
+package validation
+
+import "github.com/qdm12/gluetun/internal/models"
+
+func WindscribeRegionChoices(servers []models.WindscribeServer) (choices []string) {
+	choices = make([]string, len(servers))
+	for i := range servers {
+		choices[i] = servers[i].Region
+	}
+	return makeUnique(choices)
+}
+
+func WindscribeCityChoices(servers []models.WindscribeServer) (choices []string) {
+	choices = make([]string, len(servers))
+	for i := range servers {
+		choices[i] = servers[i].City
+	}
+	return makeUnique(choices)
+}
+
+func WindscribeHostnameChoices(servers []models.WindscribeServer) (choices []string) {
+	choices = make([]string, len(servers))
+	for i := range servers {
+		choices[i] = servers[i].Hostname
+	}
+	return makeUnique(choices)
+}