Keep the colleague's microbu-esp32c5 tree in this repository

obu-firmware builds against vanetza-idf from microbu-esp32c5/external, but
that tree was gitignored, so a clone of this repository could not build the
firmware it ships. It is now committed here as ordinary files in its own
folder, microbu-esp32c5/: the colleague's commit cf4b99f plus the V2X2MAP
bridge's signature verification (--trust) used on the bench. Nothing is
fetched from or pushed to the colleague's repository; this repository and
its remotes carry everything. The folder's own .gitignore keeps build output,
downloaded components and private key material out, as it did there; the
committed file set is identical to that repository's tracked files.

The ESP32-C5 is still flashed from obu-firmware/, which only takes
vanetza-idf from microbu-esp32c5/, so the two stay separate folders.
FLASHING.md says how to take a newer version of the colleague's tree (copy
it over the folder, rebuild, test, commit).

microbu-esp32c5/external/vanetza-idf/vanetza/security/v2/tests/recipient_info.cpp

@@ -0,0 +1,25 @@
+#include <gtest/gtest.h>
+#include <vanetza/common/byte_sequence.hpp>
+#include <vanetza/security/v2/recipient_info.hpp>
+#include <vanetza/security/v2/tests/check_recipient_info.hpp>
+#include <vanetza/security/tests/serialization.hpp>
+
+using namespace vanetza;
+using namespace vanetza::security;
+using namespace vanetza::security::v2;
+
+TEST(RecipientInfo, Serialize)
+{
+    EciesEncryptedKey ecies;
+    ecies.v = Compressed_Lsb_Y_0 { random_byte_sequence(field_size(PublicKeyAlgorithm::ECIES_NISTP256), 1337) };
+    auto cbuf = random_byte_sequence(field_size(SymmetricAlgorithm::AES128_CCM), 7331);
+    ecies.c = { cbuf.begin(), cbuf.end() };
+    auto tbuf = random_byte_sequence(ecies.t.size(), 1234);
+    std::copy_n(tbuf.begin(), ecies.t.size(), ecies.t.data());
+    RecipientInfo info;
+    info.enc_key = ecies;
+    auto certbuf = random_byte_sequence(info.cert_id.size(), 4321);
+    std::copy_n(certbuf.begin(), info.cert_id.size(), info.cert_id.data());
+
+    check(info, serialize_roundtrip(info, SymmetricAlgorithm::AES128_CCM));
+}