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/secrets.go

@@ -51,22 +51,22 @@ func (r *reader) getFromEnvOrSecretFile(envKey string, compulsory bool, retroKey
 	file, fileErr := os.OpenFile(filepath, os.O_RDONLY, 0)
 	if os.IsNotExist(fileErr) {
 		if compulsory {
-			return "", envErr
+			return "", fmt.Errorf("environment variable %s: %w", envKey, envErr)
 		}
 		return "", nil
 	} else if fileErr != nil {
-		return "", fmt.Errorf("%w: %s", ErrReadSecretFile, fileErr)
+		return "", fmt.Errorf("%w: %s: %s", ErrReadSecretFile, filepath, fileErr)
 	}
 
 	b, err := io.ReadAll(file)
 	if err != nil {
-		return "", fmt.Errorf("%w: %s", ErrReadSecretFile, err)
+		return "", fmt.Errorf("%w: %s: %s", ErrReadSecretFile, filepath, err)
 	}
 
 	value = string(b)
 	value = cleanSuffix(value)
 	if compulsory && value == "" {
-		return "", ErrSecretFileIsEmpty
+		return "", fmt.Errorf("%s: %w", filepath, ErrSecretFileIsEmpty)
 	}
 
 	return value, nil