Add world force

[caguero]

🎉 New feature

Summary

This PR extends the Link class with the ability to apply a force with an offset.

Thanks to @azeey for all the physics tips!

Test it

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 and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #1279 (b6ec8a7) into ign-gazebo6 (989cd28) will increase coverage by 0.01%.
The diff coverage is 88.88%.

@@               Coverage Diff               @@
##           ign-gazebo6    #1279      +/-   ##
===============================================
+ Coverage        62.22%   62.23%   +0.01%     
===============================================
  Files              278      278              
  Lines            23122    23131       +9     
===============================================
+ Hits             14387    14396       +9     
  Misses            8735     8735              

Impacted Files	Coverage Δ
include/ignition/gazebo/Link.hh	100.00% <ø> (ø)
src/Link.cc	93.98% <88.88%> (+0.43%)	⬆️

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[iche033]

added test in d1676c3. Marking this PR as ready.

[arjo129]

The logic looks good to me.

[osrf-triage]

This pull request has been mentioned on Gazebo Community. There might be relevant details there:

https://community.gazebosim.org/t/new-ignition-releases-2022-01-24-citadel-edifice-fortress/1241/1