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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,48 @@
+#ifndef PROXY_HEADER_HPP_JWTV3BDT
+#define PROXY_HEADER_HPP_JWTV3BDT
+
+#include <boost/range/iterator_range.hpp>
+#include <vanetza/access/access_category.hpp>
+#include <cstdint>
+
+namespace vanetza
+{
+
+#ifndef _MSC_VER
+// GCC and Clang support packed attribute
+#define PACKED_STRUCT(name, block) struct name { block } __attribute__((__packed__));
+#else
+// MSVC variant
+#define PACKED_STRUCT(name, block) \
+__pragma(pack(push, 1)) \
+struct name { block }; \
+__pragma(pack(pop))
+#endif
+
+PACKED_STRUCT(ProxyHeader,
+    /** offset in bytes to payload's first byte, counting from packet start */
+    uint16_t payload_offset;
+
+    /** length of payload in bytes */
+    uint32_t payload_size;
+
+    /** signal power in dBm, fixed point format 8.8 = trunc(double * 256) */
+    int16_t signal_power;
+
+    /** access category (transmission only) */
+    uint8_t access_category;
+)
+
+static_assert(sizeof(ProxyHeader) == 9, "ProxyHeader has invalid size");
+
+double get_signal_power(const ProxyHeader&);
+void set_signal_power(ProxyHeader&, double dbm);
+access::AccessCategory get_access_category(const ProxyHeader&);
+void set_access_category(ProxyHeader&, access::AccessCategory);
+boost::iterator_range<const uint8_t*> get_payload(const ProxyHeader&, std::size_t);
+void set_payload(ProxyHeader&, std::size_t);
+
+} // namespace vanetza
+
+#endif /* PROXY_HEADER_HPP_JWTV3BDT */
+