Add isChannelActive() function to radio interface

src/mesh/RF95Interface.h

@@ -40,6 +40,9 @@ class RF95Interface : public RadioLibInterface
      */
     virtual void enableInterrupt(void (*callback)()) { lora->setDio0Action(callback); }
 
+    /** can we detect a LoRa preamble on the current channel? */
+    virtual bool isChannelActive() override;
+
     /** are we actively receiving a packet (only called during receiving state) */
     virtual bool isActivelyReceiving() override;