Implement add_offset_array for PeriodIndex
This already works for DTI/TDI, this just rounds it out.

There will be some code de-duplication we can do if/when #19744 goes through.

lgtm. you want to de-dupliate code first? # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19826?src=pr&el=h1) Report
> Merging [#19826](https://codecov.io/gh/pandas-dev/pandas/pull/19826?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/cd1b168cf0fe12d3d850e2f98e3e7164c649f274?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `84.61%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19826/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19826?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19826      +/-   ##
==========================================
+ Coverage   91.58%   91.61%   +0.02%     
==========================================
  Files         150      150              
  Lines       48892    48905      +13     
==========================================
+ Hits        44780    44802      +22     
+ Misses       4112     4103       -9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.98% <84.61%> (+0.02%)` | :arrow_up: |
| #single | `41.78% <23.07%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19826?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/19826/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `92.87% <84.61%> (-0.17%)` | :arrow_down: |
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/19826/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `96.84% <0%> (-0.22%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19826/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.95% <0%> (+1.73%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19826?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19826?src=pr&el=footer). Last update [cd1b168...a6d9714](https://codecov.io/gh/pandas-dev/pandas/pull/19826?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 > lgtm. you want to de-dupliate code first?

If we're otherwise ready I'd rather merge and de-dup in the next pass.  My turnaround is faster than the CI's. back to you :)