feat: add business_day_count function

[MarcoGorelli]

This is the most basic case: count the number of weekdays between two dates

I'd like to suggest starting with this one, and then considering the following as follow-ups:

• add week_mask and holidays args
• add add_business_day function
• add is_business_day function

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This functionality is pretty much a port of NumPy's busday_count, so as required I've included their license. Separate to this PR, but a few other licenses should probably be included too

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status: I'm trying to sort out the structure, moving the implementation to polars-ops as that's where most functions seem to be implemented happy for this to be reviewed

[codecov]

Codecov Report

Attention: Patch coverage is 99.22481% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 81.14%. Comparing base (c211fad) to head (ff3bbd6).

❗ Current head ff3bbd6 differs from pull request most recent head 219227d. Consider uploading reports for the commit 219227d to get more accurate results

Files	Patch %	Lines
...ates/polars-plan/src/dsl/function_expr/business.rs	94.44%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15512      +/-   ##
==========================================
+ Coverage   81.12%   81.14%   +0.01%     
==========================================
  Files        1363     1368       +5     
  Lines      175161   175290     +129     
  Branches     2527     2527              
==========================================
+ Hits       142108   142244     +136     
+ Misses      32575    32568       -7     
  Partials      478      478              

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[ritchie46]

This could lead to problems as we want to distribute as MIT/Apache. But I also don't believe this required. You have interpreted their source code and written it as Rust. I think a link with credits should suffice.