feat: Convert to given time zone in .str.to_datetime when values are offset-aware

[MarcoGorelli]

related to #16297

This only deals with the .str.to_datetime part, and makes the behaviour match the constructor behaviour in https://github.com/pola-rs/polars/pull/16828/files for offset-aware input and non-missing time_zone:

In [4]: pl.Series(['1970-01-01T00:00:00+03:30']).str.to_datetime(time_zone='Iran')
Out[4]: 
shape: (1,)
Series: '' [datetime[μs, Iran]]
[
        1970-01-01 00:00:00 +0330
]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.38%. Comparing base (c81ef69) to head (dc75b4c).
Report is 17 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16742   +/-   ##
=======================================
  Coverage   81.38%   81.38%           
=======================================
  Files        1425     1425           
  Lines      187606   187662   +56     
  Branches     2697     2702    +5     
=======================================
+ Hits       152677   152732   +55     
  Misses      34434    34434           
- Partials      495      496    +1     

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[codspeed-hq]

CodSpeed Performance Report

Merging #16742 will degrade performances by 16.38%

_{Comparing MarcoGorelli:convert-to-tz (dc75b4c) with main (eaedf74)}

Summary

❌ 1 regressions
✅ 36 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	MarcoGorelli:convert-to-tz	Change
❌	test_groupby_h2oai_q3	2.4 ms	2.9 ms	-16.38%

[stinodego]

Looks good, thanks! Left a very minor nitpick but is good to merge imo.

Thanks for picking this up.

[MarcoGorelli]

thanks @stinodego ! cool, merging then