[wpilib, hal] Add function to read the RSL state (#5312)

wpilibj/src/main/java/edu/wpi/first/wpilibj/RobotController.java

@@ -103,6 +103,15 @@ public final class RobotController {
     return HAL.getBrownedOut();
   }
 
+  /**
+   * Gets the current state of the Robot Signal Light (RSL).
+   *
+   * @return The current state of the RSL- true if on, false if off
+   */
+  public static boolean getRSLState() {
+    return HAL.getRSLState();
+  }
+
   /**
    * Get the input voltage to the robot controller.
    *