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/tools/pki/pem.hpp

@@ -0,0 +1,32 @@
+#pragma once
+
+#include "keys.hpp"
+#include <vanetza/common/byte_buffer.hpp>
+#include <boost/optional/optional.hpp>
+#include <filesystem>
+#include <string>
+
+namespace vanetza
+{
+namespace pki
+{
+
+boost::optional<PrivateKey> parse_pem_private_key(const std::string&);
+boost::optional<PrivateKey> parse_pem_private_key(const ByteBuffer&);
+
+/**
+ * Encode an EC private key as unencrypted PKCS#8 PEM.
+ * Round-trips through parse_pem_private_key.
+ */
+std::string make_pem(const PrivateKey&);
+
+/**
+ * Encode `priv` as PEM and write it to `path`.
+ *
+ * \throws std::runtime_error if the file cannot be created
+ * \throws OpenSslException on write failure
+ */
+void write_pem_private_key(const PrivateKey& priv, const std::filesystem::path& path);
+
+} // namespace pki
+} // namespace vanetza