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/public_key.hpp

@@ -0,0 +1,39 @@
+#pragma once
+#include <vanetza/common/byte_buffer.hpp>
+#include <vanetza/security/key_type.hpp>
+#include <string>
+
+namespace vanetza
+{
+namespace security
+{
+
+enum class KeyCompression
+{
+    NoCompression,
+    Y0,
+    Y1
+};
+
+struct PublicKey
+{
+    ByteBuffer x;
+    ByteBuffer y;
+    KeyType type;
+    KeyCompression compression;
+};
+
+/**
+ * Compute canonical hex string of a compressed public key.
+ * \return hex string, or empty string if the key is malformed
+ */
+std::string canonical_hexstring(const PublicKey&);
+
+/**
+ * Encode public key as SubjectPublicKeyInfo according to RFC 5480.
+ * \return DER-encoded SubjectPublicKeyInfo, or empty buffer if the key is malformed
+ */
+ByteBuffer encode_subject_public_key_info(const PublicKey&);
+
+} // namespace security
+} // namespace vanetza