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/hexstring.hpp

@@ -0,0 +1,30 @@
+#pragma once
+
+#include "vanetza/common/byte_buffer.hpp"
+#include <array>
+#include <cstdint>
+#include <string>
+
+namespace vanetza
+{
+
+namespace pki
+{
+
+std::string hexstring(const std::uint8_t* buf, std::size_t len);
+std::string hexstring(const std::string& input);
+std::string hexstring(const ByteBuffer& buffer);
+
+template<size_t N> std::string hexstring(const std::array<std::uint8_t, N>& array)
+{
+    return hexstring(array.data(), array.size());
+}
+
+// true iff input is a non-empty, even-length run of hex digits
+bool is_valid_hexstring(const std::string& input);
+
+// decode hex digits to raw bytes
+std::string parse_hexstring(const std::string& input);
+
+} // namespace pki
+} // namespace vanetza