[wpilib][sim] Add Onboard IMU Sim (#8855)

This provides the ability to simulate parts of the Onboard IMU at the
HAL level. This allows team to use and simulate the IMU in code, and a
follow up PR could be made to the halsim_gui to add a new widget to view
and modify the data graphically.

Since the C++ IMU uses radians for angles that is what I did for the
simulator.

Partially deals with #8845

wpilibj/src/test/java/org/wpilib/hardware/imu/OnboardIMUTest.java

@@ -0,0 +1,59 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+package org.wpilib.hardware.imu;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+import org.wpilib.simulation.OnboardIMUSim;
+
+class OnboardIMUTest {
+  @Test
+  void testOnboardIMU() {
+    OnboardIMU imu = new OnboardIMU(OnboardIMU.MountOrientation.FLAT);
+
+    assertEquals(0.0, imu.getAngleX());
+    assertEquals(0.0, imu.getAngleY());
+    assertEquals(0.0, imu.getAngleZ());
+
+    assertEquals(0.0, imu.getGyroRateX());
+    assertEquals(0.0, imu.getGyroRateY());
+    assertEquals(0.0, imu.getGyroRateZ());
+
+    assertEquals(0.0, imu.getAccelX());
+    assertEquals(0.0, imu.getAccelY());
+    assertEquals(0.0, imu.getAccelZ());
+
+    assertEquals(0.0, imu.getYawRadians());
+
+    OnboardIMUSim.setAngleX(1);
+    OnboardIMUSim.setAngleY(2);
+    OnboardIMUSim.setAngleZ(3);
+
+    OnboardIMUSim.setGyroRateX(3.504);
+    OnboardIMUSim.setGyroRateY(1.91);
+    OnboardIMUSim.setGyroRateZ(22.9);
+
+    OnboardIMUSim.setAccelX(-1);
+    OnboardIMUSim.setAccelY(-2);
+    OnboardIMUSim.setAccelZ(-3);
+
+    OnboardIMUSim.setYaw(1.234);
+
+    assertEquals(1.0, imu.getAngleX());
+    assertEquals(2.0, imu.getAngleY());
+    assertEquals(3.0, imu.getAngleZ());
+
+    assertEquals(3.504, imu.getGyroRateX());
+    assertEquals(1.91, imu.getGyroRateY());
+    assertEquals(22.9, imu.getGyroRateZ());
+
+    assertEquals(-1.0, imu.getAccelX());
+    assertEquals(-2.0, imu.getAccelY());
+    assertEquals(-3.0, imu.getAccelZ());
+
+    assertEquals(1.234, imu.getYawRadians());
+  }
+}