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/ivpn.go

@@ -0,0 +1,37 @@
+package validation
+
+import (
+	"github.com/qdm12/gluetun/internal/models"
+)
+
+func IvpnCountryChoices(servers []models.IvpnServer) (choices []string) {
+	choices = make([]string, len(servers))
+	for i := range servers {
+		choices[i] = servers[i].Country
+	}
+	return makeUnique(choices)
+}
+
+func IvpnCityChoices(servers []models.IvpnServer) (choices []string) {
+	choices = make([]string, len(servers))
+	for i := range servers {
+		choices[i] = servers[i].City
+	}
+	return makeUnique(choices)
+}
+
+func IvpnISPChoices(servers []models.IvpnServer) (choices []string) {
+	choices = make([]string, len(servers))
+	for i := range servers {
+		choices[i] = servers[i].ISP
+	}
+	return makeUnique(choices)
+}
+
+func IvpnHostnameChoices(servers []models.IvpnServer) (choices []string) {
+	choices = make([]string, len(servers))
+	for i := range servers {
+		choices[i] = servers[i].Hostname
+	}
+	return makeUnique(choices)
+}