fix: Polars optional import

[alespour]

Closes #90

Proposed Changes

• polars module is imported only when available and actually used
• extracted PolarsDataFrameSerializer to a new file
• added test for InfluxDBClient3.write_file method for related issue verification
• added polars DataFrame serializer tests (skipped if polars is not installed)

No changes in public API.

Checklist

• Rebased/mergeable
• A test has been added if appropriate
• Tests pass
• Commit messages are conventional

[codecov]

Codecov Report

Attention: Patch coverage is 70.14925% with 20 lines in your changes missing coverage. Please review.

Project coverage is 52.84%. Comparing base (ede1956) to head (6b576df).

Files	Patch %	Lines
...client/client/write/polars_dataframe_serializer.py	70.96%	18 Missing ⚠️
influxdb_client_3/write_client/client/_base.py	50.00%	1 Missing ⚠️
influxdb_client_3/write_client/client/write_api.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #95      +/-   ##
==========================================
+ Coverage   49.75%   52.84%   +3.09%     
==========================================
  Files          32       33       +1     
  Lines        2034     2036       +2     
==========================================
+ Hits         1012     1076      +64     
+ Misses       1022      960      -62     

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[bednar]

LGTM 🚀