Add msg definitions used by micro-ROS graph manager

msg/Entity.msg

@@ -0,0 +1,8 @@
+byte PUBLISHER=0
+byte SUBSCRIBER=1
+byte SERVICE_SERVER=2
+byte SERVICE_CLIENT=3
+
+byte entity_type
+string<=256 name
+string<=256[] types

msg/Graph.msg

@@ -0,0 +1 @@
+Node[] nodes

msg/Node.msg

@@ -0,0 +1,3 @@
+string<=256 node_namespace
+string<=256 node_name
+Entity[] entities