Create Topic Statistics System Test

[dabonnie]

Description

Create a Topic Statistics system test in the ros2/system_tests repo. This should fully test the topic statistics feature and verify statistics are generated for a variety if publishers (with and without a timestamp in the message header).

Test Plan

• Manual testing

Documentation Plan

• new README.md for the demo package

Release Plan

• check in PR

Acceptance Criteria

• Code has been implemented, reviewed and merged.
• Test plan has been completed
• Release plan has been completed

Once all above items are checked, this story can be moved to done.

[prajakta-gokhale]

Topic statistics e2e test in system_metrics_collector: https://github.com/ros-tooling/system_metrics_collector/blob/master/system_metrics_collector/test/topic_statistics_collector/test_subscriber_topic_statistics.cpp

[dabonnie]

Tracking here: ros2/ros2#917