CLN: factor apply out of frame.py

[jreback]

xref #18577

[jreback]

this is a factoring out of the apply code to a separate module. Then in #18577 can refactor properly to handle the many cases.

This doesn't change any functionailty or API.

cc @jorisvandenbossche @chris-b1

[codecov]

Codecov Report

Merging #18754 into master will decrease coverage by <.01%.
The diff coverage is 99.45%.

@@            Coverage Diff             @@
##           master   #18754      +/-   ##
==========================================
- Coverage   91.61%    91.6%   -0.01%     
==========================================
  Files         153      154       +1     
  Lines       51363    51423      +60     
==========================================
+ Hits        47058    47108      +50     
- Misses       4305     4315      +10

Flag	Coverage Δ
#multiple	89.47% <99.45%> (+0.01%)	⬆️
#single	40.81% <56.83%> (-0.07%)	⬇️

Impacted Files	Coverage Δ
pandas/core/sparse/frame.py	94.8% <100%> (+0.02%)	⬆️
pandas/core/frame.py	97.68% <100%> (-0.23%)	⬇️
pandas/core/apply.py	99.42% <99.42%> (ø)
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️

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[jorisvandenbossche]

+1, looks like a good refactor!
Can you add some docstrings when working further on it?

[jreback]

yep next pr
going to rip this apart