Helper function to get an entity from an entity message

[chapulina]

🎉 New feature

• Broken off New Apply Link Wrench system #1593
• Requires Add ACTOR and WORLD types to entity gz-msgs#275

Summary

The Entity message has some documentation explaining how the message is meant to be used, but we don't have a helper function implementing that, leaving it up to plugin developers to parse the message as they see fit.

Over time, we've accumulated a couple of different implementations for the message parsing, and it would be good to standardize it in a single place. On ign-gazebo6, there are a couple of implementations in the UserCommands system:

• topLevelEntityFromMessage
• scopedEntityFromMsg

The implementation here is a combination of the 2 above, which matches the description of the message.

Test it

See the added tests.

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.

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[codecov]

Codecov Report

Merging #1595 (c2b9656) into ign-gazebo3 (c565398) will increase coverage by 0.04%.
The diff coverage is 97.36%.

❗ Current head c2b9656 differs from pull request most recent head 57c9381. Consider uploading reports for the commit 57c9381 to get more accurate results

@@               Coverage Diff               @@
##           ign-gazebo3    #1595      +/-   ##
===============================================
+ Coverage        77.76%   77.81%   +0.04%     
===============================================
  Files              250      250              
  Lines            14447    14465      +18     
===============================================
+ Hits             11235    11256      +21     
+ Misses            3212     3209       -3     

Impacted Files	Coverage Δ
include/ignition/gazebo/Util.hh	100.00% <ø> (ø)
src/Util.cc	93.97% <97.29%> (+0.57%)	⬆️
src/systems/user_commands/UserCommands.cc	82.00% <100.00%> (+0.18%)	⬆️

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