[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

wpilibcExamples/src/main/cpp/examples/DifferentialDrivePoseEstimator/cpp/Drivetrain.cpp

@@ -132,8 +132,7 @@ void Drivetrain::SimulationPeriodic() {
   leftEncoderSim.SetRate(drivetrainSimulator.GetLeftVelocity().value());
   rightEncoderSim.SetDistance(drivetrainSimulator.GetRightPosition().value());
   rightEncoderSim.SetRate(drivetrainSimulator.GetRightVelocity().value());
-  // gyroSim.SetAngle(-drivetrainSimulator.GetHeading().Degrees().value());
-  // // TODO(Ryan): fixup when sim implemented
+  imuSim.SetYaw(-drivetrainSimulator.GetHeading().Degrees());
 }
 
 void Drivetrain::Periodic() {

wpilibcExamples/src/main/cpp/examples/DifferentialDrivePoseEstimator/include/Drivetrain.hpp

@@ -23,6 +23,7 @@
 #include "wpi/nt/NetworkTableInstance.hpp"
 #include "wpi/simulation/DifferentialDrivetrainSim.hpp"
 #include "wpi/simulation/EncoderSim.hpp"
+#include "wpi/simulation/OnboardIMUSim.hpp"
 #include "wpi/smartdashboard/Field2d.hpp"
 #include "wpi/units/angle.hpp"
 #include "wpi/units/angular_velocity.hpp"
@@ -166,6 +167,7 @@ class Drivetrain {
   // Simulation classes
   wpi::sim::EncoderSim leftEncoderSim{leftEncoder};
   wpi::sim::EncoderSim rightEncoderSim{rightEncoder};
+  wpi::sim::OnboardIMUSim imuSim;
   wpi::Field2d fieldSim;
   wpi::Field2d fieldApproximation;
   wpi::math::LinearSystem<2, 2, 2> drivetrainSystem =