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FLASHING.md says how to take a newer version of the colleague's tree (copy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/asn1/srem.hpp

@@ -0,0 +1,23 @@
+#ifndef SREM_HPP_G458S63A
+#define SREM_HPP_G458S63A
+
+#include <vanetza/asn1/asn1c_conversion.hpp>
+#include <vanetza/asn1/asn1c_wrapper.hpp>
+#include <vanetza/asn1/its/SREM.h>
+
+namespace vanetza
+{
+namespace asn1
+{
+
+class Srem : public asn1c_per_wrapper<SREM_t>
+{
+public:
+    using wrapper = asn1c_per_wrapper<SREM_t>;
+    Srem() : wrapper(asn_DEF_SREM) {}
+};
+
+} // namespace asn1
+} // namespace vanetza
+
+#endif // SREM_HPP_G458S63A