Added service to set camera's follow offset

[jennuine]

🎉 New feature

Summary

Added service /gui/follow/offset to set the user camera's follow offset.

Test it

ign gazebo diff_drive.sdf

ign service -s /gui/follow --reqtype ignition.msgs.StringMsg --reptype ignition.msgs.Boolean --timeout 2000 --req 'data: "vehicle_blue"'

# sets camera to face vechile_blue
ign service -s /gui/follow/offset --reqtype ignition.msgs.Vector3d --reptype ignition.msgs.Boolean --timeout 2000 --req "x: 5, y: 0, z: 3"

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

[codecov]

Codecov Report

Merging #855 (85b559c) into ign-gazebo4 (5694336) will decrease coverage by 0.02%.
The diff coverage is 26.66%.

@@               Coverage Diff               @@
##           ign-gazebo4     #855      +/-   ##
===============================================
- Coverage        65.72%   65.69%   -0.03%     
===============================================
  Files              240      240              
  Lines            17783    17797      +14     
===============================================
+ Hits             11687    11691       +4     
- Misses            6096     6106      +10     

Impacted Files	Coverage Δ
src/gui/plugins/scene3d/Scene3D.hh	66.66% <ø> (ø)
src/gui/plugins/scene3d/Scene3D.cc	9.64% <26.66%> (+0.17%)	⬆️

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[iche033]

nice, works for me

[iche033]

while reviewing this PR, I found a small bug related to the camera follow offset checks and addressed it in #857

[nkoenig]

I'll try this out with some log files soon. Please wait for my approval before merging.