tests(dds): enable GTest smoke-tests and add async-capable realistic MockDDS with GTest async subscription test

Lts/DDS/DdsUnitTestsPlaceholder/mocks/MockDDS.h

@@ -2,21 +2,24 @@
 #include <string>
 #include <functional>
 #include <vector>
+#include <chrono>
 
 namespace MockDDS {
 
 // Richer mock for migration smoke-tests:
-// - publish(topic, payload)
+// - publish(topic, payload)                        : synchronous publish
+// - publish_async(topic, payload, delay_ms)        : asynchronous publish (background dispatcher)
 // - subscribe(topic, callback) -> subscription id
 // - unsubscribe(topic, subscription id)
 // - take_last(topic), get_queue(topic)
 // - clear_all()
-// - sequence numbers per topic (as simple integer appended to payload meta)
+// - sequence numbers per topic (integer appended to payload meta)
 
 using Subscriber = std::function<void(const std::string &payload)>;
 using SubscriptionId = unsigned long;
 
 void publish(const std::string &topic, const std::string &msg);
+void publish_async(const std::string &topic, const std::string &msg, unsigned delay_ms = 0);
 SubscriptionId subscribe(const std::string &topic, Subscriber cb);
 void unsubscribe(const std::string &topic, SubscriptionId id);
 std::string take_last(const std::string &topic);