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.

simulation/frc_gazebo_plugins/src/clockTest/world/clockTest.world

@@ -0,0 +1,17 @@
+<?xml version="1.0" ?>
+<sdf version="1.5">
+    <world name="default">
+        <scene>
+            <ambient>0.2 0.2 0.2 1</ambient>
+            <background>1 1 1 1</background>
+            <shadows>false</shadows>
+            <grid>false</grid>
+            <origin_visual>false</origin_visual>
+        </scene>
+        <model name='Clock'>
+            <plugin name='clock' filename='libclock.so'>
+                <topic>/gazebo/frc/time</topic>
+            </plugin>
+        </model>
+    </world>
+</sdf>