Add SensorTopic component to rendering sensors

[iche033]

🎉 New feature

Closes #85

Summary

The SensorTopic component is already available for non-rendering sensors (e.g. imu, force-torque, etc). This PR adds the component to rendering sensors, i.e. all sensors managed by the sensors system. The SensorTopic components stores the topic published by the sensor.

Note that rendering sensors may publish more than one topics, e.g. <prefix>/image and <prefix>/camera_info. In this case it'll only store the <prefix>/image topic.

Test it

Run the INTEGRATION_sensor_system test.

Alternative,

1. launch sensors_demo.sdf world, ign gazebo -v 4 sensors_demo.sdf
2. Go to the Entity Tree gui plugin on the right, and expand any one of the sensor models and its link, then click on a sensor entity to see the new SensorTopic component in the Component Inspector

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #1908 (b75d537) into ign-gazebo6 (29c13bf) will increase coverage by 0.03%.
The diff coverage is 96.22%.

❗ Current head b75d537 differs from pull request most recent head 66ddbe5. Consider uploading reports for the commit 66ddbe5 to get more accurate results

@@               Coverage Diff               @@
##           ign-gazebo6    #1908      +/-   ##
===============================================
+ Coverage        64.79%   64.83%   +0.03%     
===============================================
  Files              322      323       +1     
  Lines            26398    26429      +31     
===============================================
+ Hits             17105    17135      +30     
- Misses            9293     9294       +1     

Impacted Files	Coverage Δ
include/ignition/gazebo/components/Component.hh	100.00% <ø> (ø)
include/ignition/gazebo/components/Factory.hh	98.03% <95.74%> (-0.67%)	⬇️
src/ComponentFactory.cc	100.00% <100.00%> (ø)
src/rendering/RenderUtil.cc	39.70% <100.00%> (+0.15%)	⬆️

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