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FLASHING.md says how to take a newer version of the colleague's tree (copy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/common/CMakeLists.txt

@@ -0,0 +1,16 @@
+set(CXX_SOURCES
+    archives.cpp
+    byte_buffer_convertible.cpp
+    byte_buffer_sink.cpp
+    byte_buffer_source.cpp
+    byte_sequence.cpp
+    byte_view.cpp
+    clock.cpp
+    manual_runtime.cpp
+    position_fix.cpp
+    unit_interval.cpp
+)
+add_vanetza_component(common ${CXX_SOURCES})
+target_link_libraries(common PUBLIC Boost::date_time)
+
+add_test_subdirectory(tests)