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/selection.go

@@ -106,6 +106,7 @@ func (selection ServerSelection) toLines() (lines []string) {
 }
 
 type OpenVPNSelection struct {
+	ConfFile   string `json:"conf_file"`         // Custom configuration file path
 	TCP        bool   `json:"tcp"`               // UDP if TCP is false
 	CustomPort uint16 `json:"custom_port"`       // HideMyAss, Mullvad, PIA, ProtonVPN, Windscribe
 	EncPreset  string `json:"encryption_preset"` // PIA - needed to get the port number
@@ -114,6 +115,10 @@ type OpenVPNSelection struct {
 func (settings *OpenVPNSelection) lines() (lines []string) {
 	lines = append(lines, lastIndent+"OpenVPN selection:")
 
+	if settings.ConfFile != "" {
+		lines = append(lines, indent+lastIndent+"Custom configuration file: "+settings.ConfFile)
+	}
+
 	lines = append(lines, indent+lastIndent+"Protocol: "+protoToString(settings.TCP))
 
 	if settings.CustomPort != 0 {