Add TriggeredPublisher system

[azeey]

The triggered publisher system publishes a user specified message on an output topic in response to an input message that matches user specified criteria.

For example, if we want to send a msgs.Twist command to a topic in response to a msgs.Boolean input with a value of true, we'd use the following configuration

<plugin filename="libignition-gazebo-triggered_publisher-system.so" name="ignition::gazebo::systems::triggered_publisher">
  <input type="ignition.msgs.Boolean" topic="move">
    <match>
        data: "true"
    </match>
  </input>
  <output type="ignition.msgs.Twist" topic="/cmd_vel">
      linear: {x: 1.5}
  </output>
</plugin>

The system can be used to match the full input message, as shown in the previous example, or specific fields inside the message:

<plugin filename="libignition-gazebo-triggered_publisher-system.so" name="ignition::gazebo::systems::triggered_publisher">
  <input type="ignition.msgs.Vector2d" topic="/input_topic">
    <match field="x">1.0</match>
    <match field="y">2.0</match>
  </input>
  <output type="ignition.msgs.Empty" topic="/output_topic"/>
</plugin>

TODO:

• Document usage with repeated fields
• Document logic_type
• Add tutorial

[chapulina]

There are some failing tests 😉

[azeey]

I have refactored some of the code and added a new tol option to matchers. Mind taking another look @chapulina?

[chapulina]

I did another pass. Some unrelated changes sneaked into the PR, and the example is not working for me.

[chapulina]

Works great! 👍