fix: Serialize UnixTimestamp proto values as raw int64 in remote onli…

[falloficaruss]

What this PR does / why we need it:

his PR fixes a bug where UnixTimestamp feature values cause JSON serialization failures when using the remote online store.

Which issue(s) this PR fixes:

Fixes #6492

Checks

• I've made sure the tests are passing.
• My commits are signed off (git commit -s)
• My PR title follows conventional commits format

Testing Strategy

• Unit tests
• Integration tests
• Manual tests
• Testing is not required for this change

Misc

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.40%. Comparing base (6019bd6) to head (cbc45bc).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6555      +/-   ##
==========================================
+ Coverage   46.36%   46.40%   +0.03%     
==========================================
  Files         414      414              
  Lines       50072    50076       +4     
  Branches     7154     7156       +2     
==========================================
+ Hits        23218    23239      +21     
+ Misses      25231    25204      -27     
- Partials     1623     1633      +10     

Flag	Coverage Δ
go-feature-server	30.58% <ø> (ø)
python-unit	47.71% <100.00%> (+0.04%)	⬆️

Files with missing lines	Coverage Δ
sdk/python/feast/infra/online_stores/remote.py	63.85% <100.00%> (+5.62%)	⬆️

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