also send MQTT definition topic and handle knx subscription for conditional messages evaluated later

src/lib/ebus/message.h

@@ -399,6 +399,12 @@ class Message : public AttributedItem {
    */
   bool isAvailable();
 
+  /**
+   * Get the time when this (potentially conditional) message first became available.
+   * @return the time when this message first became available, or 0 if it is not available.
+   */
+  time_t getAvailableSinceTime();
+
   /**
    * Return whether the field is available.
    * @param fieldName the name of the field to find, or nullptr for any.
@@ -673,6 +679,9 @@ class Message : public AttributedItem {
   /** the @a Condition for this message, or nullptr. */
   Condition* m_condition;
 
+  /** the time when the @a Condition first became available, or 0. */
+  time_t m_availableSinceTime;
+
   /** the last seen @a MasterSymbolString. */
   MasterSymbolString m_lastMasterData;
 
@@ -927,6 +936,12 @@ class Condition {
    */
   virtual bool isTrue() = 0;
 
+  /**
+   * Get the system time when the condition was last checked.
+   * @return the system time when the condition was last checked, 0 for never.
+   */
+  time_t getLastCheckTime() const { return m_lastCheckTime; }
+
 
  protected:
   /** the system time when the condition was last checked, 0 for never. */