fix: Use microseconds for Unity catalog datetime unit

[nameexhaustion]

The type returned from deltalake indicates microseconds.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 79.28%. Comparing base (15fa50a) to head (7259b3e).
Report is 23 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-io/src/catalog/unity/schema.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21122      +/-   ##
==========================================
- Coverage   79.29%   79.28%   -0.01%     
==========================================
  Files        1589     1589              
  Lines      226515   226515              
  Branches     2594     2594              
==========================================
- Hits       179611   179600      -11     
- Misses      46308    46319      +11     
  Partials      596      596              

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