Add the monitoring system related messages

The goal is to provide messages used by the monitoring system mentioned in Autoware issue #821.

The following messages are included in this MR:

• AutowareState
• DrivingCapability
• EmergencyState
• Engage
• HazardStatus
• HazardStatusStamped

The monitoring system consists of the following components:

• Autoware State Monitor (system/autoware_state_monitor)
• Autoware Error Monitor (system/autoware_error_monitor)
• Emergency Handler (system/emergency_handler)

The architecture is shown in the below diagram (more details in issue #821).

autoware_auto_msgs/msg/AutowareState.idl

+#include "builtin_interfaces/msg/Time.idl"
+
+module autoware_auto_msgs {
+  module msg {
+    module AutowareState_Constants {
+      const uint8 INITIALIZING = 1;
+      const uint8 WAITING_FOR_ROUTE = 2;
+      const uint8 PLANNING = 3;
+      const uint8 WAITING_FOR_ENGAGE = 4;
+      const uint8 DRIVING = 5;
+      const uint8 ARRIVED_GOAL = 6;
+      const uint8 FINALIZING = 7;
+    };
+
+    @verbatim (language="comment", text=
+      " A message for reporting the Autoware system status.")
+    struct AutowareState {
+      builtin_interfaces::msg::Time stamp;
+
+      @verbatim (language="comment", text=
+        " Current state of the Autoware system.")
+      uint8 state;
+    };
+  };
+};
+