perf: Lazy gather for {forward,backward}_fill in group-by contexts

[coastalwhite]

This should significantly speed up {forward,backward}_fill and their aliases fill_null(strategy='forward'/'backward') in group-by contexts like group-by, cumulative_eval, pivot, {list,arr}.{eval,agg}, over and rolling.

[codecov]

Codecov Report

❌ Patch coverage is 99.19355% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.77%. Comparing base (f72d1ea) to head (a85f0fd).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-expr/src/dispatch/groups_dispatch.rs	99.16%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25115      +/-   ##
==========================================
+ Coverage   81.75%   81.77%   +0.02%     
==========================================
  Files        1707     1707              
  Lines      235616   235740     +124     
  Branches     3005     3005              
==========================================
+ Hits       192633   192786     +153     
+ Misses      42215    42186      -29     
  Partials      768      768              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.