Speed up Resource Spawner load time by fetching model list asynchronously

[azeey]

🦟 Bug fix

Fixes #1253

Summary

The main reason that the Resource Spawner took a long time to load is that it tried to fetch the list of all available models on Fuel instead of just the selected owner. And it did that while blocking the Qt thread, so the user was unable to interact with the GUI while the model list was being fetched. The approach I've taken is to only fetch the list of models for the default owner ("openrobotics") and allow users to add/remove any owner they want. The fetching is also done in a separate thread so as to not block the GUI.

This currently does not preserve the list of owners added by the user. I thought it would be better to add that in a follow up PR at a future time.

Needs gazebosim/gz-fuel-tools#350

Checklist

• Signed all commits for DCO
• Added tests
• 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.

[mjcarroll]

This one has conflicts now.

[mjcarroll]

Great improvement. LGTM with green CI!

[codecov]

Codecov Report

Merging #1962 (fa22f51) into ign-gazebo3 (a2a2c85) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

❗ Current head fa22f51 differs from pull request most recent head 606f209. Consider uploading reports for the commit 606f209 to get more accurate results

@@               Coverage Diff               @@
##           ign-gazebo3    #1962      +/-   ##
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- Coverage        78.04%   78.03%   -0.02%     
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  Files              255      255              
  Lines            15080    15082       +2     
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  Hits             11769    11769              
- Misses            3311     3313       +2     

Impacted Files	Coverage Δ
src/gz.cc	63.88% <0.00%> (-0.72%)	⬇️