Add stub offsets to smargon

[DominicOram]

Needed for DiamondLightSource/hyperion#794

Instructions to reviewer on how to test:

1. Confirm unit tests pass

Checks for reviewer

• Would the PR title make sense to a scientist on a set of release notes
• If a new device has been added does it follow the standards

[codecov]

Codecov Report

Merging #230 (b8f9eab) into main (8498a43) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #230      +/-   ##
==========================================
+ Coverage   88.06%   88.19%   +0.12%     
==========================================
  Files          72       72              
  Lines        2555     2583      +28     
==========================================
+ Hits         2250     2278      +28     
  Misses        305      305              

Files	Coverage Δ
src/dodal/devices/smargon.py	100.00% <100.00%> (ø)
src/dodal/devices/status.py	100.00% <100.00%> (ø)
src/dodal/devices/utils.py	100.00% <100.00%> (ø)

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[dperl-dls]

Looks good!