chore(rust!): move offset_by implementation from polars-plan to polars-time, rename feature from DateOffset to OffsetBy

[MarcoGorelli]

addressing a comment from a previous pr #16728 (comment)

I've also:

• renamed FunctionExpr::DateOffset to FunctionExpr::TemporalExpr(TemporalFunction::OffsetBy
• renamed the feature date_offset to offset_by

I've noticed that whereas in polars-ops, every feature seems to be under its own feature flag, in polars-time that doesn't seem to be the case. I've put offset_by, month_start, and month_end under their own feature flags - is this ok?

Functional changes: none

[codspeed-hq]

CodSpeed Performance Report

Merging #16796 will not alter performance

_{Comparing MarcoGorelli:move-offset-by (be8bd6b) with main (fa12a8e)}

Summary

✅ 37 untouched benchmarks

[codecov]

Codecov Report

Attention: Patch coverage is 93.54839% with 6 lines in your changes missing coverage. Please review.

Project coverage is 81.37%. Comparing base (38149d6) to head (be8bd6b).
Report is 1 commits behind head on main.

Files	Patch %	Lines
crates/polars-time/src/offset_by.rs	95.06%	4 Missing ⚠️
...ates/polars-plan/src/dsl/function_expr/datetime.rs	80.00%	1 Missing ⚠️
py-polars/src/lazyframe/visitor/expr_nodes.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16796      +/-   ##
==========================================
- Coverage   81.39%   81.37%   -0.02%     
==========================================
  Files        1424     1425       +1     
  Lines      187623   187599      -24     
  Branches     2691     2691              
==========================================
- Hits       152719   152667      -52     
- Misses      34409    34437      +28     
  Partials      495      495              

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[ritchie46]

offset_by, month_start, and month_end under their own feature flags - is this ok?

Yes, that's ok. That saves compile times/bloat for rust users, who don't need that.