priv impl and scaffold for driver

include/bbnow_pub.hpp

@@ -7,6 +7,7 @@
 #include <vector>
 #include <optional>
 #include <span>
+#include <variant>
 
 #include "bb_expantions.hpp"
 
@@ -30,10 +31,11 @@ enum class BBNowPubPacketType: uint16_t {
     MAX = 1,
 };
 
-/// The data field for the packet
-union BlkBoxNowPubPacketData {
-    BlkBoxNowPubDiscoveryData discovery;
-};
+/// The data field for the packet.
+using BlkBoxNowPubPacketData = std::variant<
+    std::monostate,
+    BlkBoxNowPubDiscoveryData
+>;
 
 struct BBNowPubPacket {
     // "bbnp" in ascii