Centralization of tdigest aggregation code.

[nvdbaranec]

This PR is prep-work for adding support for invoking the tdigest aggregations through cudf::reduce. It doesn't change any code, just moves some things around:

• groupby/sort/group_tdigest.cu -> quantiles/tdigest/tdigest_aggregation.cu
• Moves exposure of the aggregation functions from group_reductions.hpp to quantiles/tdigest/tdigest.hpp

Doing this as a seperate PR so the actual changes for the reduction work show up as a reasonable diff.

[vyasr]

CMake approval.

[codereport]

👍

[vuule]

LGTM
Can't tell what's wrong with CMake style :\

[ttnghia]

cmake-format failed ;)

[nvdbaranec]

@gpucibot merge

[codecov]

Codecov Report

Merging #10422 (e79e3e6) into branch-22.04 (b613394) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head e79e3e6 differs from pull request most recent head 25c1849. Consider uploading reports for the commit 25c1849 to get more accurate results

@@               Coverage Diff                @@
##           branch-22.04   #10422      +/-   ##
================================================
+ Coverage         86.13%   86.15%   +0.01%     
================================================
  Files               139      139              
  Lines             22460    22438      -22     
================================================
- Hits              19347    19332      -15     
+ Misses             3113     3106       -7     

Impacted Files	Coverage Δ
python/cudf/cudf/io/parquet.py	92.70% <ø> (ø)
python/cudf/cudf/core/column/column.py	89.16% <100.00%> (+0.17%)	⬆️
python/cudf/cudf/core/dataframe.py	93.57% <100.00%> (ø)
python/cudf/cudf/core/frame.py	91.72% <100.00%> (ø)
python/cudf/cudf/core/groupby/groupby.py	91.57% <100.00%> (+0.08%)	⬆️
python/cudf/cudf/core/single_column_frame.py	97.01% <100.00%> (ø)
python/cudf/cudf/core/column/string.py	88.39% <0.00%> (+0.12%)	⬆️
python/cudf/cudf/core/tools/datetimes.py	84.49% <0.00%> (+0.30%)	⬆️
python/cudf/cudf/core/column/lists.py	90.56% <0.00%> (+0.47%)	⬆️

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