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FLASHING.md says how to take a newer version of the colleague's tree (copy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