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FLASHING.md says how to take a newer version of the colleague's tree (copy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/dcc/burst_budget.cpp

@@ -0,0 +1,71 @@
+#include <vanetza/common/runtime.hpp>
+#include <vanetza/dcc/burst_budget.hpp>
+#include <cassert>
+
+namespace vanetza
+{
+namespace dcc
+{
+
+namespace
+{
+// these constants are given in C2C-CC Basic System Profile (last v1.3.0)
+constexpr Clock::duration T_Burst = std::chrono::seconds(1);
+constexpr Clock::duration T_BurstPeriod = std::chrono::seconds(10);
+constexpr std::size_t N_Burst = 20;
+} // namespace
+
+
+BurstBudget::BurstBudget(const Runtime& rt) :
+    m_runtime(rt), m_messages(N_Burst), m_burst_duration(T_Burst), m_burst_period(T_BurstPeriod)
+{
+}
+
+BurstBudget::~BurstBudget()
+{
+}
+
+Clock::duration BurstBudget::delay()
+{
+    assert(m_burst_duration < m_burst_period);
+    Clock::duration delay = Clock::duration::max();
+
+    if (m_messages.empty()) {
+        delay = Clock::duration::zero();
+    } else if (m_messages.front() + m_burst_duration > m_runtime.now() && !m_messages.full()) {
+        delay = Clock::duration::zero();
+    } else if (m_messages.front() + m_burst_period < m_runtime.now()) {
+        m_messages.clear();
+        delay = Clock::duration::zero();
+    } else {
+        delay = m_messages.front() + m_burst_period - m_runtime.now();
+    }
+
+    return delay;
+}
+
+void BurstBudget::notify()
+{
+    m_messages.push_back(m_runtime.now());
+}
+
+void BurstBudget::burst_messages(std::size_t n)
+{
+    // circular_buffer::resize removes last elements if necessary
+    m_messages.resize(n);
+}
+
+void BurstBudget::burst_duration(Clock::duration d)
+{
+    assert(d > Clock::duration::zero());
+    m_burst_duration = d;
+}
+
+void BurstBudget::burst_period(Clock::duration p)
+{
+    assert(p > Clock::duration::zero());
+    m_burst_period = p;
+}
+
+} // namespace dcc
+} // namespace vanetza