VPNSP value custom for OpenVPN custom config files (#621)

- Retro-compatibility: `OPENVPN_CUSTOM_CONFIG` set implies `VPNSP=custom`
- Change: `up` and `down` options are not filtered out
- Change: `OPENVPN_INTERFACE` overrides the network interface defined in the configuration file
- Change: `PORT` overrides any port found in the configuration file
- Feat: config file is read when building the OpenVPN configuration, so it's effectively reloaded on VPN restarts
- Feat: extract values from custom file at start to log out valid settings
- Maint: `internal/openvpn/extract` package instead of `internal/openvpn/custom` package
- Maint: All providers' `BuildConf` method return an error
- Maint: rename `CustomConfig` to `ConfFile` in Settings structures

internal/configuration/openvpn.go

@@ -21,7 +21,7 @@ type OpenVPN struct {
 	Root      bool     `json:"run_as_root"`
 	Cipher    string   `json:"cipher"`
 	Auth      string   `json:"auth"`
-	Config    string   `json:"custom_config"`
+	ConfFile  string   `json:"conf_file"`
 	Version   string   `json:"version"`
 	ClientCrt string   `json:"-"`                 // Cyberghost
 	ClientKey string   `json:"-"`                 // Cyberghost, VPNUnlimited
@@ -59,8 +59,8 @@ func (settings *OpenVPN) lines() (lines []string) {
 		lines = append(lines, indent+lastIndent+"Custom auth algorithm: "+settings.Auth)
 	}
 
-	if len(settings.Config) > 0 {
-		lines = append(lines, indent+lastIndent+"Custom configuration: "+settings.Config)
+	if settings.ConfFile != "" {
+		lines = append(lines, indent+lastIndent+"Configuration file: "+settings.ConfFile)
 	}
 
 	if settings.ClientKey != "" {
@@ -83,13 +83,14 @@ func (settings *OpenVPN) lines() (lines []string) {
 }
 
 func (settings *OpenVPN) read(r reader, serviceProvider string) (err error) {
-	settings.Config, err = r.env.Get("OPENVPN_CUSTOM_CONFIG", params.CaseSensitiveValue())
-	if err != nil {
-		return fmt.Errorf("environment variable OPENVPN_CUSTOM_CONFIG: %w", err)
+	credentialsRequired := false
+	switch serviceProvider {
+	case constants.Custom:
+	case constants.VPNUnlimited:
+	default:
+		credentialsRequired = true
 	}
 
-	credentialsRequired := settings.Config == "" && serviceProvider != constants.VPNUnlimited
-
 	settings.User, err = r.getFromEnvOrSecretFile("OPENVPN_USER", credentialsRequired, []string{"USER"})
 	if err != nil {
 		return fmt.Errorf("environment variable OPENVPN_USER: %w", err)
@@ -159,6 +160,8 @@ func (settings *OpenVPN) read(r reader, serviceProvider string) (err error) {
 	}
 
 	switch serviceProvider {
+	case constants.Custom:
+		err = settings.readCustom(r) // read OPENVPN_CUSTOM_CONFIG
 	case constants.Cyberghost:
 		err = settings.readCyberghost(r)
 	case constants.PrivateInternetAccess: