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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,51 @@
+#pragma once
+
+#include <vanetza/common/byte_buffer.hpp>
+#include <vanetza/security/key_type.hpp>
+#include <vanetza/security/private_key.hpp>
+#include <vanetza/security/public_key.hpp>
+#include <vanetza/security/signature.hpp>
+
+// forward declarations
+struct Vanetza_Security_Signature;
+struct Vanetza_Security_PublicVerificationKey;
+struct Vanetza_Security_PublicEncryptionKey;
+
+namespace vanetza
+{
+namespace pki
+{
+
+using security::KeyCompression;
+using security::KeyType;
+using security::PrivateKey;
+using security::PublicKey;
+using security::Signature;
+
+PublicKey derive_public_key(const PrivateKey&);
+Signature make_signature(const struct Vanetza_Security_Signature&);
+
+/**
+ * Generate a fresh random EC private key on the curve matching `type`.
+ *
+ * \throws OpenSslException on generation failure
+ */
+PrivateKey generate_private_key(KeyType type);
+
+/**
+ * Populate an ASN.1 PublicVerificationKey CHOICE from a PublicKey.
+ *
+ * \throws std::invalid_argument on unsupported key types
+ */
+void set_verification_key(struct Vanetza_Security_PublicVerificationKey&, const PublicKey&);
+
+/**
+ * Populate an ASN.1 PublicEncryptionKey from a PublicKey.
+ * Sets supportedSymmAlg to AES-128-CCM.
+ *
+ * \throws std::invalid_argument on unsupported key types
+ */
+void set_encryption_key(struct Vanetza_Security_PublicEncryptionKey&, const PublicKey&);
+
+} // namespace pki
+} // namespace vanetza