[hal] Add function for changing HAL Notifier thread priority (#3218)

wpilibc/src/main/native/include/frc/Notifier.h

@@ -143,6 +143,23 @@ class Notifier : public ErrorBase {
    */
   void Stop();
 
+  /**
+   * Sets the HAL notifier thread priority.
+   *
+   * The HAL notifier thread is responsible for managing the FPGA's notifier
+   * interrupt and waking up user's Notifiers when it's their time to run.
+   * Giving the HAL notifier thread real-time priority helps ensure the user's
+   * real-time Notifiers, if any, are notified to run in a timely manner.
+   *
+   * @param realTime Set to true to set a real-time priority, false for standard
+   *                 priority.
+   * @param priority Priority to set the thread to. For real-time, this is 1-99
+   *                 with 99 being highest. For non-real-time, this is forced to
+   *                 0. See "man 7 sched" for more details.
+   * @return         True on success.
+   */
+  static bool SetHALThreadPriority(bool realTime, int32_t priority);
+
  private:
   /**
    * Update the HAL alarm time.