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/custom.go

@@ -0,0 +1,53 @@
+package configuration
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/qdm12/gluetun/internal/constants"
+	"github.com/qdm12/golibs/params"
+)
+
+var (
+	errCustomNotSupported    = errors.New("custom provider is not supported")
+	errCustomExtractFromFile = errors.New("cannot extract configuration from file")
+)
+
+func (settings *Provider) readCustom(r reader, vpnType string) (err error) {
+	settings.Name = constants.Custom
+
+	if vpnType != constants.OpenVPN {
+		return fmt.Errorf("%w: for VPN type %s", errCustomNotSupported, vpnType)
+	}
+
+	return settings.readCustomOpenVPN(r)
+}
+
+func (settings *Provider) readCustomOpenVPN(r reader) (err error) {
+	configFile, err := r.env.Get("OPENVPN_CUSTOM_CONFIG", params.CaseSensitiveValue(), params.Compulsory())
+	if err != nil {
+		return fmt.Errorf("environment variable OPENVPN_CUSTOM_CONFIG: %w", err)
+	}
+	settings.ServerSelection.OpenVPN.ConfFile = configFile
+
+	// For display and consistency purposes only,
+	// these values are not actually used since the file is re-read
+	// before each OpenVPN start.
+	_, connection, err := r.ovpnExt.Data(configFile)
+	if err != nil {
+		return fmt.Errorf("%w: %s", errCustomExtractFromFile, err)
+	}
+	settings.ServerSelection.OpenVPN.TCP = connection.Protocol == constants.TCP
+
+	return nil
+}
+
+func (settings *OpenVPN) readCustom(r reader) (err error) {
+	settings.ConfFile, err = r.env.Path("OPENVPN_CUSTOM_CONFIG",
+		params.Compulsory(), params.CaseSensitiveValue())
+	if err != nil {
+		return fmt.Errorf("environment variable OPENVPN_CUSTOM_CONFIG: %w", err)
+	}
+
+	return nil
+}