implement Delegator class
This follows #17651 in breaking #17042 into smaller pieces.  I expect one more piece after this, but will consider it a win if it ends here.

At the moment `PandasDelegate` defines `_create_delegator_property` and `_create_delegator_method` inside `_add_delegate_accessors` (but namespace-wise do not need to).  When first trying to figure out how `str`, `cat`, and `dt` worked, I found the levels of redirection confusing.  This PR

a) moves these dynamically-defined functions into staticmethods of a new `Delegator` class,

b) fleshes out the relationship between `PandasDelegate` and `AccessorProperty` in docstrings,

c) provides a new `wrap_delegate_names` class decorator as an alternative to `PandasDelegate._add_delegate_accessors` for pinning specifying the delegated properties/methods,

d) implements tests for custom accessors.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

BTW the git diff is broken up in an unfortunate way.  Is there a way to tell show it where it should start and stop the diff blocks? # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17662?src=pr&el=h1) Report
> Merging [#17662](https://codecov.io/gh/pandas-dev/pandas/pull/17662?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6da85b30d989855fe2a1f5d1323189f0fc639e60?src=pr&el=desc) will **decrease** coverage by `0.03%`.
> The diff coverage is `73.52%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17662/graphs/tree.svg?src=pr&width=650&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17662?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17662      +/-   ##
==========================================
- Coverage   91.26%   91.23%   -0.04%     
==========================================
  Files         163      163              
  Lines       49806    49820      +14     
==========================================
- Hits        45455    45451       -4     
- Misses       4351     4369      +18
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.02% <73.52%> (-0.02%)` | :arrow_down: |
| #single | `40.3% <70.58%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17662?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/accessor.py](https://codecov.io/gh/pandas-dev/pandas/pull/17662?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYWNjZXNzb3IucHk=) | `83.11% <73.52%> (-10.54%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17662?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17662?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.73% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17662?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17662?src=pr&el=footer). Last update [6da85b3...81ac134](https://codecov.io/gh/pandas-dev/pandas/pull/17662?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17662?src=pr&el=h1) Report
> Merging [#17662](https://codecov.io/gh/pandas-dev/pandas/pull/17662?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6da85b30d989855fe2a1f5d1323189f0fc639e60?src=pr&el=desc) will **decrease** coverage by `0.03%`.
> The diff coverage is `73.52%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17662/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17662?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17662      +/-   ##
==========================================
- Coverage   91.26%   91.23%   -0.04%     
==========================================
  Files         163      163              
  Lines       49806    49820      +14     
==========================================
- Hits        45455    45451       -4     
- Misses       4351     4369      +18
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.02% <73.52%> (-0.02%)` | :arrow_down: |
| #single | `40.3% <70.58%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17662?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/accessor.py](https://codecov.io/gh/pandas-dev/pandas/pull/17662?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYWNjZXNzb3IucHk=) | `83.11% <73.52%> (-10.54%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17662?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17662?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.73% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17662?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17662?src=pr&el=footer). Last update [6da85b3...81ac134](https://codecov.io/gh/pandas-dev/pandas/pull/17662?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 note really sure how this improves things. The motivation is very much based around clarity for newcomers.  I had trouble figuring out `PandasDelegate`/`AccessorProperty` early on when I wanted to implement a custom accessor; the idea is to clarify that for the next person to try it.  But if that's not a customization that we want to expose, then this can be ignored.

Docstrings/exposition/tests aside, the only code-organization improvement here is removing an unnecessary level of nesting in the implementations of `_create_delegator_property` and `_create_delegator_method`.

This is also a small step in the direction of implementing xarray's custom accessor api. this is not anymore clear IMHO. you can actually do this using proper descriptors, that would be a nice change. This is not a battle worth winning.  Closing.