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FLASHING.md says how to take a newer version of the colleague's tree (copy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/v3/sign_service.cpp

@@ -0,0 +1,62 @@
+#include <vanetza/common/its_aid.hpp>
+#include <vanetza/common/runtime.hpp>
+#include <vanetza/security/v2/basic_elements.hpp>
+#include <vanetza/security/backend.hpp>
+#include <vanetza/security/v3/hash.hpp>
+#include <vanetza/security/v3/sign_service.hpp>
+#include <vanetza/security/v3/secured_message.hpp>
+
+namespace vanetza
+{
+namespace security
+{
+namespace v3
+{
+
+StraightSignService::StraightSignService(CertificateProvider& provider, Backend& backend, SignHeaderPolicy& policy, CertificateValidator& validator) :
+    m_certificates(provider), m_backend(backend), m_policy(policy), m_validator(validator)
+{
+}
+
+SignConfirm StraightSignService::sign(SignRequest&& request)
+{
+    const auto& signing_cert = m_certificates.own_certificate();
+    const auto hash_algo = specified_hash_algorithm(signing_cert.get_verification_key_type());
+
+    SecuredMessage secured_message = SecuredMessage::with_signed_data();
+    secured_message.set_hash_id(hash_algo);
+    secured_message.set_payload(convert_to_payload(request.plain_message));
+    m_policy.prepare_header(request, secured_message);
+
+    if (m_validator.valid_for_signing(signing_cert, request.its_aid) != CertificateValidator::Verdict::Valid) {
+        return SignConfirm::failure(SignConfirmError::No_Certificate);
+    }
+
+    ByteBuffer digest = calculate_message_hash(m_backend, hash_algo, secured_message.signing_payload(), signing_cert);
+    Signature signature = m_backend.sign_digest(m_certificates.own_private_key(), digest);
+    secured_message.set_signature(signature);
+    return SignConfirm::success(std::move(secured_message));
+}
+
+DummySignService::DummySignService(const Runtime& runtime) :
+    m_runtime(runtime)
+{
+}
+
+SignConfirm DummySignService::sign(SignRequest&& request)
+{
+    SecuredMessage secured_message = SecuredMessage::with_signed_data();
+    ByteBuffer payload;
+    payload = convert_to_payload(request.plain_message);
+    secured_message.set_payload(payload);
+    secured_message.set_dummy_signature();
+    secured_message.set_its_aid(request.its_aid);
+    secured_message.set_generation_time(vanetza::security::v2::convert_time64(m_runtime.now()));
+    secured_message->content->choice.signedData->signer.present = Vanetza_Security_SignerIdentifier_PR_self;
+
+    return SignConfirm::success(std::move(secured_message));
+}
+
+} // namespace v3
+} // namespace security
+} // namespace vanetza