AP_HAL: added time_shift() scheduler API

used for log replay

libraries/AP_HAL/Scheduler.h

@@ -43,6 +43,11 @@ public:
        optional function to set timer speed in Hz
      */
     virtual void     set_timer_speed(uint16_t speed_hz) {}
+
+    /**
+       optional function to shift forward in time, used by log replay
+     */
+    virtual void     time_shift(uint32_t shift_ms) {}
 };
 
 #endif // __AP_HAL_SCHEDULER_H__