Add support for custom sensors

[chapulina]

🎉 New feature

Part of gazebosim/gz-sensors#9

Summary

Creating custom sensors that aren't officially part of the spec shouldn't be disallowed. This PR provides a simple way for users to tell SDF that they're loading a custom sensor, so it's not an error if SDF doesn't recognize the sensor type. Note that this is different from a sensor with NONE type, which is an error.

Users are encouraged to use Custom SDF Elements to pass configuration to their custom sensors, like the ignition:custom_sensor element below:

        <sensor name="custom_sensor" type="custom" ignition:type="custom_sensor">
          <always_on>1</always_on>
          <update_rate>30</update_rate>
          <visualize>true</visualize>
          <topic>custom</topic>
          <ignition:custom_sensor>
            <noise type="gaussian">
              <mean>0.2</mean>
              <stddev>0.1</stddev>
            </noise>
          </ignition:custom_sensor>
        </sensor>

This is an initial proposal, I'm open to other ideas for supporting custom sensors.

Test it

I'm making use of this on these WIP PRs, you can try them out:

• Remove plugin interface and support custom sensors gz-sensors#90
• Upgrade ign-sensors and support custom sensors gz-sim#617

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial (downstream)
• Updated documentation (as needed)
• Updated migration guide (as needed)
• 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

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[codecov-commenter]

Codecov Report

Merging #652 (84b5009) into main (22a968e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #652   +/-   ##
=======================================
  Coverage   87.94%   87.94%           
=======================================
  Files          72       72           
  Lines       10891    10893    +2     
=======================================
+ Hits         9578     9580    +2     
  Misses       1313     1313           

Impacted Files	Coverage Δ
src/Sensor.cc	80.35% <100.00%> (+0.17%)	⬆️

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[caguero]

Looks good to me!