Keep vanetza-idf in obu-firmware, so a plain clone builds the firmware

obu-firmware builds against the vanetza-idf C-ITS library, which until now
came from the colleague's microbu-esp32c5 tree beside the repository and was
not tracked here, so a clone of this repository could not build the firmware
it ships. The library alone is now part of obu-firmware, as
obu-firmware/external/vanetza-idf: their external/vanetza-idf at commit
cf4b99f, unchanged (9775 files; see its PROVENANCE.md). CMake takes it from
there by default; -DVANETZA_IDF_DIR still points the build elsewhere.

The rest of the colleague's tree (their own VAM firmware, PKI tooling,
station-link Python tools, the V2X2MAP bridge) stays out of this repository
and gitignored; nothing is pushed to their repository. NOTES.md, docs/06,
TODO.md and the pcap verifier's usage line point at the new location.

obu-firmware/external/vanetza-idf/vanetza/security/persistence.hpp

@@ -0,0 +1,35 @@
+#pragma once
+#include <vanetza/security/private_key.hpp>
+#include <string>
+
+namespace vanetza
+{
+namespace security
+{
+
+/**
+ * Load a private key from a PEM file, deriving its type from the stored curve.
+ * \param key_path PEM file to load the key from
+ * \return loaded private key
+ */
+PrivateKey load_private_key_from_pem_file(const std::string& key_path);
+
+/**
+ * Load a private key from a DER file, deriving its type from the stored curve.
+ * \param key_path DER file to load the key from
+ * \return loaded private key
+ */
+PrivateKey load_private_key_from_der_file(const std::string& key_path);
+
+#ifdef VANETZA_WITH_OPENSSL
+PrivateKey load_private_key_from_pem_file_openssl(const std::string& key_path);
+PrivateKey load_private_key_from_der_file_openssl(const std::string& key_path);
+#endif
+
+#ifdef VANETZA_WITH_CRYPTOPP
+PrivateKey load_private_key_from_pem_file_cryptopp(const std::string& key_path);
+PrivateKey load_private_key_from_der_file_cryptopp(const std::string& key_path);
+#endif
+
+} // namespace security
+} // namespace vanetza