CLN: factor apply out of frame.py
xref #18577 
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](https://codecov.io/gh/pandas-dev/pandas/pull/18754?src=pr&el=h1) Report
> Merging [#18754](https://codecov.io/gh/pandas-dev/pandas/pull/18754?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9705a480615201ce1e89e5385f48fde155726e8b?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `99.45%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18754/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18754?src=pr&el=tree)

```diff
@@            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%)` | :arrow_up: |
| #single | `40.81% <56.83%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18754?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/sparse/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18754/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3BhcnNlL2ZyYW1lLnB5) | `94.8% <100%> (+0.02%)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18754/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.68% <100%> (-0.23%)` | :arrow_down: |
| [pandas/core/apply.py](https://codecov.io/gh/pandas-dev/pandas/pull/18754/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXBwbHkucHk=) | `99.42% <99.42%> (ø)` | |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18754/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |

------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18754?src=pr&el=footer). Last update [9705a48...0f5e6ae](https://codecov.io/gh/pandas-dev/pandas/pull/18754?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 +1, looks like a good refactor! 
Can you add some docstrings when working further on it? yep next pr 
going to rip this apart