Drop pandas dependency, switch to narwhals for df-agnosticity

[MarcusRisanger]

Fixes #37 by replacing pandas as a core dependency with narwhals.

Can now use polars dataframes (and any dataframes supported by narwhals) in DataverseEntity.upsert and DataverseEntity.create.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.97%. Comparing base (75dbb50) to head (9103f82).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #38      +/-   ##
==========================================
+ Coverage   91.13%   91.97%   +0.83%     
==========================================
  Files          17       17              
  Lines         869      885      +16     
==========================================
+ Hits          792      814      +22     
+ Misses         77       71       -6     

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