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/openvpn/extract/read.go

@@ -0,0 +1,27 @@
+package extract
+
+import (
+	"io"
+	"os"
+	"strings"
+)
+
+func readCustomConfigLines(filepath string) (
+	lines []string, err error) {
+	file, err := os.Open(filepath)
+	if err != nil {
+		return nil, err
+	}
+
+	b, err := io.ReadAll(file)
+	if err != nil {
+		_ = file.Close()
+		return nil, err
+	}
+
+	if err := file.Close(); err != nil {
+		return nil, err
+	}
+
+	return strings.Split(string(b), "\n"), nil
+}