implement maybe_wrap_in_index for EA Mixin dispatch
Implements `maybe_wrap_in_index`, shows the easiest places where it can be used.  If merged, the follow-on will be to move the affected methods to the EA mixin classes, leaving just the dispatch/wrapping behind.

Implements `_local_timestamps` on the `DatetimeArrayMixin`, since that will be needed before we can move the comparison operators among others.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21715?src=pr&el=h1) Report
> Merging [#21715](https://codecov.io/gh/pandas-dev/pandas/pull/21715?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8f322149ebd2a30f2f2d48c06863cd783166fa84?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `94.11%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21715/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/21715?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21715      +/-   ##
==========================================
+ Coverage   91.92%   91.93%   +<.01%     
==========================================
  Files         158      158              
  Lines       49703    49708       +5     
==========================================
+ Hits        45689    45698       +9     
+ Misses       4014     4010       -4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <94.11%> (ø)` | :arrow_up: |
| #single | `42% <38.23%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21715?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/21715/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `100% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/21715/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `92.57% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/21715/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.45% <100%> (-0.04%)` | :arrow_down: |
| [pandas/core/indexes/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/21715/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy90aW1lZGVsdGFzLnB5) | `91.37% <100%> (+0.17%)` | :arrow_up: |
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/21715/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `97.32% <81.81%> (+0.6%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21715?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/21715?src=pr&el=footer). Last update [8f32214...4c6f25e](https://codecov.io/gh/pandas-dev/pandas/pull/21715?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Part of this PR is superseded by #21721. The other part is in limbo. Closing to clear the queue.
