Implement add_offset_array for PeriodIndex

[jbrockmendel]

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.

[jreback]

lgtm. you want to de-dupliate code first?

[codecov]

Codecov Report

Merging #19826 into master will increase coverage by 0.02%.
The diff coverage is 84.61%.

@@            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%)	⬆️
#single	41.78% <23.07%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pandas/core/indexes/period.py	92.87% <84.61%> (-0.17%)	⬇️
pandas/core/indexes/datetimelike.py	96.84% <0%> (-0.22%)	⬇️
pandas/plotting/_converter.py	66.95% <0%> (+1.73%)	⬆️

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[jbrockmendel]

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.

[jreback]

back to you :)