[REVIEW] Add dictionary support to cudf::quantile

[davidwendt]

Reference #5963

Add support for dictionary column types in cudf::quantile. This function is also used by cudf::reduce in PR #6666.
The code change here is simply to dispatch on the dictionary key type and then pass a dictionary-iterator to transform function that computes the quantile values -- select_quantile_data.
This means this PR is also dependent on #6651 for the is_dictionary trait.

Also, added a simple gtest for testing this new dictionary code path.

[codecov]

Codecov Report

Merging #6676 (2ad86da) into branch-0.17 (88821fb) will increase coverage by 0.06%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           branch-0.17    #6676      +/-   ##
===============================================
+ Coverage        82.21%   82.27%   +0.06%     
===============================================
  Files               94       94              
  Lines            15478    15539      +61     
===============================================
+ Hits             12725    12785      +60     
- Misses            2753     2754       +1     

Impacted Files	Coverage Δ
python/cudf/cudf/core/column/lists.py	92.74% <0.00%> (+5.44%)	⬆️

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[rgsl888prabhu]

LGTM