fix: Use Buffer<T> in ObjectSeries, fixes variety of offset bugs

[orlp]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 6 lines in your changes missing coverage. Please review.

Project coverage is 79.91%. Comparing base (72d861e) to head (2422d39).
Report is 50 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-core/src/chunked_array/object/mod.rs	75.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18637      +/-   ##
==========================================
- Coverage   79.93%   79.91%   -0.03%     
==========================================
  Files        1506     1506              
  Lines      203058   203031      -27     
  Branches     2891     2891              
==========================================
- Hits       162312   162247      -65     
- Misses      40196    40234      +38     
  Partials      550      550              

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