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/security_entity.hpp

@@ -0,0 +1,43 @@
+#ifndef SECURITY_ENTITY_HPP
+#define SECURITY_ENTITY_HPP
+
+#include <vanetza/security/decap_service.hpp>
+#include <vanetza/security/encap_service.hpp>
+
+namespace vanetza
+{
+namespace security
+{
+
+class SecurityEntity
+{
+public:
+    /**
+     * \brief Creates a security envelope covering the given payload.
+     *
+     * The payload consists of the CommonHeader, ExtendedHeader and the payload of
+     * the layers above the network layer. The entire security envelope is used
+     * to calculate a signature which gets added to the resulting SecuredMessage.
+     *
+     * \param request containing payload to sign
+     * \return confirmation containing signed SecuredMessage
+     */
+    virtual EncapConfirm encapsulate_packet(EncapRequest&& request) = 0;
+
+    /**
+     * \brief Decapsulates the payload within a SecuredMessage
+     *
+     * Verifies the Signature and SignerInfo of a SecuredMessage.
+     *
+     * \param request containing a SecuredMessage
+     * \return decapsulation confirmation including plaintext payload
+     */
+    virtual DecapConfirm decapsulate_packet(DecapRequest&& request) = 0;
+
+    virtual ~SecurityEntity() = default;
+};
+
+} // namespace security
+} // namespace vanetza
+
+#endif // SECURITY_ENTITY_HPP