fix(rust): Make casting to date timezone-aware

[florian-klein]

Context

This fixes #22864

DateTime stores the timestamp in unix time, which leads to inconsistencies when timezones are used.
This for example leads to an incorrect cast to date, which previously acted as if no timezone was present at all.

I used chrono to implement the parsing - let me know if there's a more idiomatic approach to this that doesn't add to much boilerplate to polars.

[MarcoGorelli]

thanks for your pr! just left a couple of comments

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.62%. Comparing base (5888845) to head (1f876d7).
Report is 33 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #22888      +/-   ##
==========================================
- Coverage   81.01%   80.62%   -0.40%     
==========================================
  Files        1671     1677       +6     
  Lines      236927   222321   -14606     
  Branches     2792     2801       +9     
==========================================
- Hits       191949   179241   -12708     
+ Misses      44309    42412    -1897     
+ Partials      669      668       -1     

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[MarcoGorelli]

thanks for updating

i think there's a function that you can use to do this whole part (replace_time_zone(..., None))?

[florian-klein]

Yep you're right, that's also where I got the logic from.

However, replace_time_zone is in polar-ops, which depends on polars-core.

Then datetime.rs is in polars-core, so we'd have a circular dependency when we'd use something from polars-ops in polars-core.