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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,39 @@
+#pragma once
+#include <vanetza/common/byte_buffer.hpp>
+#include <vanetza/security/key_type.hpp>
+#include <string>
+
+namespace vanetza
+{
+namespace security
+{
+
+enum class KeyCompression
+{
+    NoCompression,
+    Y0,
+    Y1
+};
+
+struct PublicKey
+{
+    ByteBuffer x;
+    ByteBuffer y;
+    KeyType type;
+    KeyCompression compression;
+};
+
+/**
+ * Compute canonical hex string of a compressed public key.
+ * \return hex string, or empty string if the key is malformed
+ */
+std::string canonical_hexstring(const PublicKey&);
+
+/**
+ * Encode public key as SubjectPublicKeyInfo according to RFC 5480.
+ * \return DER-encoded SubjectPublicKeyInfo, or empty buffer if the key is malformed
+ */
+ByteBuffer encode_subject_public_key_info(const PublicKey&);
+
+} // namespace security
+} // namespace vanetza