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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,44 @@
+#ifndef HEADER_CONVERSION_HPP_SQVEUMFE
+#define HEADER_CONVERSION_HPP_SQVEUMFE
+
+#include <vanetza/btp/header.hpp>
+#include <vanetza/common/byte_buffer_convertible.hpp>
+#include <vanetza/common/serialization_buffer.hpp>
+
+namespace vanetza
+{
+namespace convertible
+{
+
+template<>
+struct byte_buffer_impl<btp::HeaderA> : public byte_buffer
+{
+    byte_buffer_impl(const btp::HeaderA& header) : m_header(header) {}
+    void convert(ByteBuffer& buffer) const override
+    {
+        buffer.clear();
+        serialize_into_buffer(m_header, buffer);
+    }
+    std::size_t size() const override { return btp::HeaderA::length_bytes; }
+
+    const btp::HeaderA m_header;
+};
+
+template<>
+struct byte_buffer_impl<btp::HeaderB> : public byte_buffer
+{
+    byte_buffer_impl(const btp::HeaderB& header) : m_header(header) {}
+    void convert(ByteBuffer& buffer) const override
+    {
+        buffer.clear();
+        serialize_into_buffer(m_header, buffer);
+    }
+    std::size_t size() const override { return btp::HeaderB::length_bytes; }
+
+    const btp::HeaderB m_header;
+};
+
+} // namespace convertible
+} // namespace vanetza
+
+#endif /* HEADER_CONVERSION_HPP_SQVEUMFE */