New service for adding systems to an entity

[iche033]

🎉 New feature

• Related issue: Insert system plugins from the GUI #190
• Depends on: Add EntityPlugin_V msg gz-msgs#261
• Builds on top of Use sdf plugin #1352

Summary

Adds a service for adding systems / plugins to an entity at run time. The service accepts the new EntityPlugin_V.proto msg introduced in gazebosim/gz-msgs#261 that contains the entity id and an array of plugins / systems to add to the entity.

Test it

Run the INTEGRATION_entity_system test. It adds the diff_drive plugin to a vehicle model and verifies that it starts moving after the plugin is added.

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.

[chapulina]

Did a first pass, the functionality works for me. Tested manually like this, for some fun results:

ign gazebo shapes.sdf -v 4 -r &
ign service -s /world/shapes/entity/system/add --reqtype ignition.msgs.EntityPlugin_V --reptype ignition.msgs.Boolean --timeout 3000 --req 'entity: {id: 1}, plugins: {name: "ignition::gazebo::systems::Buoyancy", filename: "ignition-gazebo-buoyancy-system"}'

[chapulina]

LGTM, let's just wait for #1352 to be merged first.

We also need an ign-msgs8 release with gazebosim/gz-msgs#261, I started a forward-port so we get more into the release: gazebosim/gz-msgs#264

[codecov]

Codecov Report

Merging #1524 (efef477) into ign-gazebo6 (ccbda48) will increase coverage by 0.00%.
The diff coverage is 83.33%.

❗ Current head efef477 differs from pull request most recent head 2ba4994. Consider uploading reports for the commit 2ba4994 to get more accurate results

@@             Coverage Diff              @@
##           ign-gazebo6    #1524   +/-   ##
============================================
  Coverage        63.96%   63.96%           
============================================
  Files              317      317           
  Lines            25584    25613   +29     
============================================
+ Hits             16364    16383   +19     
- Misses            9220     9230   +10     

Impacted Files	Coverage Δ
src/SimulationRunner.cc	90.93% <69.23%> (-1.11%)	⬇️
src/SystemManager.cc	97.08% <89.65%> (-2.92%)	⬇️

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