Maint: internal/openvpn/parse package

- Parse PEM key data for Cyberghost and VPNUnlimited
- Add more unit tests

internal/openvpn/parse/key.go

@@ -0,0 +1,14 @@
+package parse
+
+import (
+	"fmt"
+)
+
+func ExtractPrivateKey(b []byte) (keyData string, err error) {
+	keyData, err = extractPEM(b, "PRIVATE KEY")
+	if err != nil {
+		return "", fmt.Errorf("%w: %s", ErrExtractPEM, err)
+	}
+
+	return keyData, nil
+}