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/include/MockData/EncoderData.h

@@ -17,6 +17,7 @@ extern "C" {
 #endif
 
 void HALSIM_ResetEncoderData(int32_t index);
+int16_t HALSIM_GetDigitalChannelA(int32_t index);
 int32_t HALSIM_RegisterEncoderInitializedCallback(int32_t index,
                                                   HAL_NotifyCallback callback,
                                                   void* param,