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/provider/custom/connection.go

@@ -0,0 +1,38 @@
+package custom
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/qdm12/gluetun/internal/configuration"
+	"github.com/qdm12/gluetun/internal/constants"
+	"github.com/qdm12/gluetun/internal/models"
+	"github.com/qdm12/gluetun/internal/provider/utils"
+)
+
+var (
+	ErrVPNTypeNotSupported = errors.New("VPN type not supported for custom provider")
+	ErrExtractConnection   = errors.New("cannot extract connection")
+)
+
+// GetConnection gets the connection from the OpenVPN configuration file.
+func (p *Provider) GetConnection(selection configuration.ServerSelection) (
+	connection models.Connection, err error) {
+	if selection.VPN != constants.OpenVPN {
+		return connection, fmt.Errorf("%w: %s", ErrVPNTypeNotSupported, selection.VPN)
+	}
+
+	_, connection, err = p.extractor.Data(selection.OpenVPN.ConfFile)
+	if err != nil {
+		return connection, fmt.Errorf("%w: %s", ErrExtractConnection, err)
+	}
+
+	connection.Port = getPort(connection.Port, selection)
+
+	return connection, nil
+}
+
+// Port found is overridden by custom port set with `PORT` or `WIREGUARD_PORT`.
+func getPort(foundPort uint16, selection configuration.ServerSelection) (port uint16) {
+	return utils.GetPort(selection, foundPort, foundPort, foundPort)
+}