Support particle_emitters in the websocket server

[nkoenig]

🎉 New feature

Summary

Adds a call to service provided by ParticleEmitter2 in the websocket plugin. This call supports returning the list of particle emitters to a client, such as gz3d.

This functionality will be deprecated in Ignition Fortress. Users will be able to rely on the Scene message from Ignition Fortress on.

This is part of a series of pull requests designed to support particle emitter in gz3d.
See also:

• Added particle_emitter vector message gz-msgs#149
• Particle emitter based on SDF gz-sim#730
• Particle emitter sdformat#528

Test it

Run the web application using both:

1. https://gitlab.com/ignitionrobotics/web/app/-/merge_requests/113
2. https://gitlab.com/ignitionrobotics/web/gz3d/-/merge_requests/44

Using websocket.ign and fog.sdf from https://gist.github.com/nkoenig/cd3f5cbf69be97f04190e3cfaa155423:

1. ign launch -v 4 websocket.ign
2. To to the /visualize route on the web app, and click connect.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• 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

[iche033]

works for me, I was able to see the particles in the web app

[codecov]

Codecov Report

Merging #104 (301073d) into ign-launch3 (88c2355) will not change coverage.
The diff coverage is n/a.

❗ Current head 301073d differs from pull request most recent head 952ae1b. Consider uploading reports for the commit 952ae1b to get more accurate results

@@             Coverage Diff              @@
##           ign-launch3     #104   +/-   ##
============================================
  Coverage        30.24%   30.24%           
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  Files                3        3           
  Lines              734      734           
============================================
  Hits               222      222           
  Misses             512      512           

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