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/vpn.go

@@ -58,10 +58,8 @@ func (settings *VPN) read(r reader) (err error) {
 	}
 	settings.Type = vpnType
 
-	if !settings.isOpenVPNCustomConfig(r.env) {
-		if err := settings.Provider.read(r, vpnType); err != nil {
-			return fmt.Errorf("%w: %s", errReadProviderSettings, err)
-		}
+	if err := settings.Provider.read(r, vpnType); err != nil {
+		return fmt.Errorf("%w: %s", errReadProviderSettings, err)
 	}
 
 	switch settings.Type {
@@ -79,19 +77,3 @@ func (settings *VPN) read(r reader) (err error) {
 
 	return nil
 }
-
-func (settings VPN) isOpenVPNCustomConfig(env params.Interface) (ok bool) {
-	if settings.Type != constants.OpenVPN {
-		return false
-	}
-	s, err := env.Get("OPENVPN_CUSTOM_CONFIG")
-	return err == nil && s != ""
-}
-
-func (settings VPN) VPNInterface() (intf string) {
-	if settings.Type == constants.Wireguard {
-		return settings.Wireguard.Interface
-	}
-	// OpenVPN
-	return settings.OpenVPN.Interface
-}