fix bug in Timestamp.add

[ajtmccarty]

instantiate the new timestamp as self.__class__ instead of Timestamp so that we keep the correct class

for example, infrahub has its own Timestamp class that inherites from the sdk's Timestamp and calling .add on an infrahub Timestamp yields an sdk Timestamp

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

@@            Coverage Diff             @@
##           stable     #403      +/-   ##
==========================================
+ Coverage   73.80%   75.12%   +1.32%     
==========================================
  Files          92       93       +1     
  Lines        8548     8560      +12     
  Branches     1680     1678       -2     
==========================================
+ Hits         6309     6431     +122     
+ Misses       1793     1676     -117     
- Partials      446      453       +7     

Flag	Coverage Δ
integration-tests	34.42% <0.00%> (+9.02%)	⬆️
python-3.10	47.50% <100.00%> (+0.20%)	⬆️
python-3.11	47.50% <100.00%> (+0.20%)	⬆️
python-3.12	47.50% <100.00%> (+0.22%)	⬆️
python-3.13	47.52% <100.00%> (+0.22%)	⬆️
python-3.9	45.79% <100.00%> (+0.11%)	⬆️
python-filler-3.12	24.26% <0.00%> (+0.03%)	⬆️

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Files with missing lines	Coverage Δ
infrahub_sdk/timestamp.py	82.35% <100.00%> (ø)

... and 8 files with indirect coverage changes

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[dgarros]

Looks good,
I think we should do the same change for subtract as well just below add

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