Maint: refactor VPN configuration structure

- Paves the way for Wireguard
- VPN struct contains Type, Openvpn and Provider configurations
- OpenVPN specific options (e.g. client key) moved from Provider to Openvpn configuration struct
- Move Provider configuration from OpenVPN configuration to VPN
- HTTP control server returns only openvpn settings (not provider settings)

internal/configuration/vpn.go

@@ -0,0 +1,71 @@
+package configuration
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+
+	"github.com/qdm12/gluetun/internal/constants"
+	"github.com/qdm12/golibs/params"
+)
+
+type VPN struct {
+	Type     string   `json:"type"`
+	OpenVPN  OpenVPN  `json:"openvpn"`
+	Provider Provider `json:"provider"`
+}
+
+func (settings *VPN) String() string {
+	return strings.Join(settings.lines(), "\n")
+}
+
+func (settings *VPN) lines() (lines []string) {
+	lines = append(lines, lastIndent+"VPN:")
+
+	lines = append(lines, indent+lastIndent+"Type: "+settings.Type)
+
+	for _, line := range settings.OpenVPN.lines() {
+		lines = append(lines, indent+line)
+	}
+
+	for _, line := range settings.Provider.lines() {
+		lines = append(lines, indent+line)
+	}
+
+	return lines
+}
+
+var (
+	errReadProviderSettings = errors.New("cannot read provider settings")
+	errReadOpenVPNSettings  = errors.New("cannot read OpenVPN settings")
+)
+
+func (settings *VPN) read(r reader) (err error) {
+	vpnType, err := r.env.Inside("VPN_TYPE",
+		[]string{constants.OpenVPN}, params.Default(constants.OpenVPN))
+	if err != nil {
+		return fmt.Errorf("environment variable VPN_TYPE: %w", err)
+	}
+	settings.Type = vpnType
+
+	if !settings.isOpenVPNCustomConfig(r.env) {
+		if err := settings.Provider.read(r); err != nil {
+			return fmt.Errorf("%w: %s", errReadProviderSettings, err)
+		}
+	}
+
+	err = settings.OpenVPN.read(r, settings.Provider.Name)
+	if err != nil {
+		return fmt.Errorf("%w: %s", errReadOpenVPNSettings, err)
+	}
+
+	return nil
+}
+
+func (settings VPN) isOpenVPNCustomConfig(env params.Env) (ok bool) {
+	if settings.Type != constants.OpenVPN {
+		return false
+	}
+	s, err := env.Get("OPENVPN_CUSTOM_CONFIG")
+	return err == nil && s != ""
+}