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/provider.go

@@ -0,0 +1,19 @@
+package custom
+
+import (
+	"github.com/qdm12/gluetun/internal/constants"
+	"github.com/qdm12/gluetun/internal/openvpn/extract"
+	"github.com/qdm12/gluetun/internal/provider/utils"
+)
+
+type Provider struct {
+	extractor extract.Interface
+	utils.NoPortForwarder
+}
+
+func New() *Provider {
+	return &Provider{
+		extractor:       extract.New(),
+		NoPortForwarder: utils.NewNoPortForwarding(constants.Custom),
+	}
+}