[ros2_control_demos] Add package.xml for metapackage.

ros2_control_demos/package.xml

+<?xml version="1.0"?>
+<package format="3">
+  <name>ros2_control_demos</name>
+  <version>0.0.0</version>
+  <description>Metapackage for ROS2 control demos packages</description>
+  <maintainer email="allabana@amazon.com">Anas Abou Allaban</maintainer>
+  <maintainer email="bence.magyar.robotics@gmail.com">Bence Magyar</maintainer>
+  <maintainer email="jordan.palacios@pal-robotics.com">Jordan Palacios</maintainer>
+  <maintainer email="karsten@openrobotics.org">Karsten Knese</maintainer>
+
+  <license>Apache License 2.0</license>
+
+  <buildtool_depend>ament_cmake</buildtool_depend>
+
+  <exec_depend>ros2_control_demos_headless</exec_depend>
+  <exec_depend>controller_manager</exec_depend>
+  <exec_depend>controller_parameter_server</exec_depend>
+  <exec_depend>hardware_interface</exec_depend>
+
+  <export>
+    <build_type>ament_cmake</build_type>
+  </export>
+</package>