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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,47 @@
+#pragma once
+
+#include "keys.hpp"
+#include "signed_data.hpp"
+#include <vanetza/common/byte_buffer.hpp>
+#include <vanetza/security/hash_algorithm.hpp>
+#include <cstdint>
+
+namespace vanetza
+{
+namespace pki
+{
+
+class Certificate;
+class SecurityModule;
+
+using security::HashAlgorithm;
+
+// TS 103 097 protocol version that we emit on every constructed EtsiTs103097Data.
+inline constexpr std::uint8_t ieee1609dot2_protocol_version = 3;
+
+/**
+ * Build an EtsiTs103097Data-Signed envelope carrying `payload` as inline
+ * `unsecuredData`, signed with `signing_key`. PSID is fixed to SCR.
+ *
+ * \param signer_cert selects the SignerIdentifier variant per IEEE 1609.2 §5.3.1:
+ *   nullptr  → signer = self    (hash of signer is hash of empty string)
+ *   non-null → signer = digest  with HashedId8(cert); hash of signer is hash
+ *              of the OER-encoded signer certificate.
+ */
+SignedData create_signed(const ByteBuffer& payload, SecurityModule& security,
+    const PublicKey& signing_key, HashAlgorithm hash_algo, const Certificate* signer_cert);
+
+/**
+ * Build an EtsiTs103097Data-SignedExternalPayload (TS 102 941 §6.2.3.3.1) used
+ * for the EC-signed proof inside an AT request: SignedData with
+ * `extDataHash = hash_algo(external_payload)` and no inline data.
+ *
+ * The same `hash_algo` covers both the external-payload hash placed in
+ * extDataHash and the signature hash chain. Other parameters mirror
+ * create_signed(); PSID is fixed to SCR.
+ */
+SignedData create_external_signed(const ByteBuffer& external_payload, SecurityModule& security,
+    const PublicKey& signing_key, HashAlgorithm hash_algo, const Certificate* signer_cert);
+
+} // namespace pki
+} // namespace vanetza