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/signature.hpp

@@ -0,0 +1,55 @@
+#ifndef CF4C0740_EE9F_493A_A3F5_95DA691E8989
+#define CF4C0740_EE9F_493A_A3F5_95DA691E8989
+
+#include <vanetza/common/byte_buffer.hpp>
+#include <vanetza/security/ecc_point.hpp>
+#include <vanetza/security/key_type.hpp>
+#include <cstddef>
+#include <future>
+
+namespace vanetza
+{
+namespace security
+{
+
+/// EcdsaSignature specified in TS 103 097 v1.2.1, section 4.2.9
+struct EcdsaSignature
+{
+    EccPoint R;
+    ByteBuffer s;
+};
+
+class EcdsaSignatureFuture
+{
+public:
+    EcdsaSignatureFuture(std::shared_future<EcdsaSignature>, EcdsaSignature placholder);
+
+    const EcdsaSignature& get() const;
+    std::size_t size() const;
+
+private:
+    mutable std::shared_future<EcdsaSignature> m_future;
+    EcdsaSignature m_placeholder;
+};
+
+using SomeEcdsaSignature = boost::variant<EcdsaSignature, EcdsaSignatureFuture>;
+
+struct Signature
+{
+    ByteBuffer r;
+    ByteBuffer s;
+    KeyType type;
+};
+
+/**
+ * \brief Extracts binary signature
+ * \param signature source for binary signature
+ * \return signature as binary
+ */
+ByteBuffer extract_signature_buffer(const SomeEcdsaSignature& sig);
+ByteBuffer extract_signature_buffer(const Signature& sig);
+
+} // namespace security
+} // namespace vanetza
+
+#endif /* CF4C0740_EE9F_493A_A3F5_95DA691E8989 */