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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,79 @@
+#ifndef SIGN_SERVICE_HPP_4MDQBSEF
+#define SIGN_SERVICE_HPP_4MDQBSEF
+
+#include <vanetza/common/byte_buffer.hpp>
+#include <vanetza/common/its_aid.hpp>
+#include <vanetza/common/position_provider.hpp>
+#include <vanetza/net/packet.hpp>
+#include <vanetza/security/hash_algorithm.hpp>
+#include <vanetza/security/key_type.hpp>
+#include <vanetza/security/secured_message.hpp>
+#include <vanetza/security/signing_policy.hpp>
+#include <boost/optional/optional.hpp>
+#include <functional>
+
+namespace vanetza
+{
+
+// forward declaration
+class Runtime;
+
+namespace security
+{
+
+// mandatory SN-SIGN.request parameters
+struct SignRequest
+{
+    DownPacket plain_message;
+    ItsAid its_aid;
+    ByteBuffer permissions;
+    // optional SN-ENCAP.request context_information (TS 102 723-8 V1.1.1 Table 24):
+    // opaque octets for selecting properties of the security protocol
+    ByteBuffer context_information;
+    bool external_payload = false;
+    bool self_signed = false;
+};
+
+enum class SignConfirmError
+{
+    Unspecified,
+    No_Certificate,
+    No_Service,
+};
+
+// mandatory SN-SIGN.confirm parameters
+struct SignConfirm
+{
+    SignConfirm(SignConfirmError error, boost::optional<SecuredMessage> message)
+        : error(error), secured_message(std::move(message))
+    {
+    }
+
+    static SignConfirm success(SecuredMessage&& message)
+    {
+        return { SignConfirmError::Unspecified, std::move(message) };
+    }
+
+    static SignConfirm failure(SignConfirmError error)
+    {
+        return { error, boost::none };
+    }
+
+    SignConfirmError error;
+    boost::optional<SecuredMessage> secured_message;
+};
+
+/**
+ * Equivalant of SN-SIGN service in TS 102 723-8 v1.1.1
+ */
+class SignService
+{
+public:
+    virtual ~SignService() = default;
+    virtual SignConfirm sign(SignRequest&&) = 0;
+};
+
+} // namespace security
+} // namespace vanetza
+
+#endif /* SIGN_SERVICE_HPP_4MDQBSEF */