[hal] Add HAL_Shutdown and simulation shutdown callbacks

This is useful for clean shutdown of simulation extensions.

hal/src/main/native/cpp/jni/HAL.cpp

@@ -36,6 +36,18 @@ Java_edu_wpi_first_hal_HAL_initialize
   return HAL_Initialize(timeout, mode);
 }
 
+/*
+ * Class:     edu_wpi_first_hal_HAL
+ * Method:    shutdown
+ * Signature: ()V
+ */
+JNIEXPORT void JNICALL
+Java_edu_wpi_first_hal_HAL_shutdown
+  (JNIEnv*, jclass)
+{
+  return HAL_Shutdown();
+}
+
 /*
  * Class:     edu_wpi_first_hal_HAL
  * Method:    hasMain