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/gz_msgs/src/main/proto/bool.proto

@@ -0,0 +1,15 @@
+package gazebo.msgs;
+
+/// \ingroup gazebo_msgs
+/// \interface Bool
+/// \brief A message for boolean data
+/// \verbatim
+
+option java_outer_classname = "GzBool";
+
+message Bool
+{
+  required bool data = 1;
+}
+
+/// \endverbatim

simulation/gz_msgs/src/main/proto/driver-station.proto

@@ -0,0 +1,21 @@
+package gazebo.msgs;
+
+/// \ingroup gazebo_msgs
+/// \interface DriverStation
+/// \brief A message for DriverStation data
+/// \verbatim
+
+option java_outer_classname = "GzDriverStation";
+
+message DriverStation
+{
+  required bool enabled = 1;
+  enum State {
+    AUTO = 0;
+    TELEOP = 1;
+    TEST = 2;
+  }
+  required State state = 2;
+}
+
+/// \endverbatim

simulation/gz_msgs/src/main/proto/float64.proto

@@ -0,0 +1,15 @@
+package gazebo.msgs;
+
+/// \ingroup gazebo_msgs
+/// \interface Float64
+/// \brief A message for floating point data
+/// \verbatim
+
+option java_outer_classname = "GzFloat64";
+
+message Float64
+{
+  required double data = 1;
+}
+
+/// \endverbatim

simulation/gz_msgs/src/main/proto/frc_joystick.proto

@@ -0,0 +1,16 @@
+package gazebo.msgs;
+
+/// \ingroup gazebo_msgs
+/// \interface Joystick
+/// \brief A message for joystick data
+/// \verbatim
+
+option java_outer_classname = "GzFRCJoystick";
+
+message FRCJoystick
+{
+  repeated double axes = 1;
+  repeated bool buttons = 2;
+}
+
+/// \endverbatim