add trailing microsecond 0s to serialized timestamps

[ajtmccarty]

IFC-1374

the new Timestamp using whenever strips trailing zeros from serialized instances. This can cause our database-level datetime comparison logic to break b/c we use string comparison on timestamp fields and a later timestamp with no trailing 0s can appear as earlier incorrectly

small cypher example
╒════════════════════════════════════════════════════════╕
│"2025-03-12T16:33:54.7Z" > "2025-03-12T16:33:54.710000Z"│
╞════════════════════════════════════════════════════════╡
│true │
└────────────────────────────────────────────────────────┘

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

@@                 Coverage Diff                  @@
##           infrahub-develop     #301      +/-   ##
====================================================
+ Coverage             70.56%   70.92%   +0.36%     
====================================================
  Files                    86       87       +1     
  Lines                  7867     7879      +12     
  Branches               1523     1527       +4     
====================================================
+ Hits                   5551     5588      +37     
+ Misses                 1920     1895      -25     
  Partials                396      396              

Flag	Coverage Δ
integration-tests	22.70% <0.00%> (+0.93%)	⬆️
python-3.10	45.23% <100.00%> (+0.07%)	⬆️
python-3.11	45.20% <100.00%> (+0.04%)	⬆️
python-3.12	45.20% <100.00%> (+0.07%)	⬆️
python-3.13	45.23% <100.00%> (+0.09%)	⬆️
python-3.9	44.10% <100.00%> (+0.07%)	⬆️
python-filler-3.12	24.57% <0.00%> (+0.03%)	⬆️

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Files with missing lines	Coverage Δ
infrahub_sdk/timestamp.py	82.35% <100.00%> (+0.35%)	⬆️

... and 6 files with indirect coverage changes

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