feat: add support for dictionary encoding from Arrow

[agoose77]

This function is intended to replace ak.to_categorical, which is now deprecated. It is not called by the same name as the Arrow function that implements it, because the Awkward parlance is "convert to categorical", and I think we want our ak.str API to be Awkward-looking, even if it uses Arrow under the hood.

[codecov]

Codecov Report

Merging #2630 (43de707) into main (6c52131) will increase coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

Files Changed	Coverage Δ
src/awkward/operations/ak_from_categorical.py	95.23% <ø> (ø)
src/awkward/operations/ak_to_categorical.py	93.54% <100.00%> (+0.21%)	⬆️
src/awkward/operations/str/__init__.py	98.82% <100.00%> (+0.01%)	⬆️
src/awkward/operations/str/akstr_to_categorical.py	100.00% <100.00%> (ø)

[jpivarski]

Great!

And I see that the deprecation is already in the wiki.

This is ready to merge.