Bring back the gazebo plugins (#1063)

The models and meshes are not included. We will need
to find an alternate way to reintegrate these and use them.

* Add simulation/gz_msgs back, and build with Gradle.

* Add back in the frc simulation plugins for gazebo.

* Add a new shared library, halsim_gazebo.

This library will become the interface between the
HAL sim layer and gazebo.

* Preserve the first channel number used in created Encoders in the Sim MockData.

This allows us to use the DIO channel number to connect with simulated encoders.

* Have the HAL Simulator set the reverse direction on creation.

This enables a simulator to be aware of the direction.

* Add a drive_motor plugin.

This is a bit of a 'magic' motor, which allows us to build robot
models that drive in a more realistic fashion.  It does this
by apply forces directly to the chassis, rather than relying on
the complex motion dynamics of a driven wheel.

This in turn allows the model to reduce wheel friction,
reducing scrub, and allowing for a more natural driving experience.

hal/src/main/native/sim/MockData/EncoderDataInternal.h

@@ -19,6 +19,9 @@
 namespace hal {
 class EncoderData {
  public:
+  void SetDigitalChannelA(int16_t channel);
+  int16_t GetDigitalChannelA() const;
+
   int32_t RegisterInitializedCallback(HAL_NotifyCallback callback, void* param,
                                       HAL_Bool initialNotify);
   void CancelInitializedCallback(int32_t uid);
@@ -86,6 +89,7 @@ class EncoderData {
 
  private:
   wpi::mutex m_registerMutex;
+  std::atomic<int16_t> m_digitalChannelA{0};
   std::atomic<HAL_Bool> m_initialized{false};
   std::shared_ptr<NotifyListenerVector> m_initializedCallbacks = nullptr;
   std::atomic<int32_t> m_count{0};