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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,57 @@
+#pragma once
+
+#include <vanetza/common/clock.hpp>
+#include <boost/date_time/posix_time/posix_time_types.hpp>
+#include <boost/optional/optional.hpp>
+#include <chrono>
+#include <string>
+
+namespace vanetza
+{
+namespace pki
+{
+
+/**
+ * Get TAI offset relative to UTC for a particular time point t
+ * \param t offset at this time point
+ * \return TAI offset in seconds
+ */
+boost::posix_time::time_duration utc_tai_offset(boost::posix_time::ptime t);
+
+/**
+ * Get current time from system clock, corrected by TAI offset
+ */
+Clock::time_point current_time();
+
+/**
+ * Parse a human-readable duration into whole hours.
+ *
+ * Accepted form: "<N><unit>" where N is a non-negative integer and unit is
+ * one of:
+ *   s — seconds (truncated to whole hours)
+ *   m — minutes (truncated to whole hours)
+ *   h — hours
+ *   d — days   (24 h)
+ *   w — weeks  (168 h)
+ *
+ * Returns boost::none on empty input, missing/unknown unit, negative value,
+ * unparseable number, or trailing garbage.
+ */
+boost::optional<std::chrono::hours> parse_duration_hours(const std::string&);
+
+/**
+ * Parse a validity-start specification.
+ *
+ * Two accepted forms:
+ *   - Relative: "+<N><unit>" — interpreted as `current_time() + duration`,
+ *     where the duration follows the same grammar as parse_duration_hours().
+ *   - Absolute (UTC): an ISO-extended date "YYYY-MM-DD" (00:00:00 implied),
+ *     or a date+time "YYYY-MM-DDTHH:MM:SS" / "YYYY-MM-DD HH:MM:SS".
+ *
+ * Returns boost::none on empty input, malformed date, or unrecognised
+ * relative duration.
+ */
+boost::optional<Clock::time_point> parse_validity_start(const std::string&);
+
+} // namespace pki
+} // namespace vanetza