[REVIEW] Add nvtext::edit_distance API

[davidwendt]

This is also to help unblock NVIDIA/cuml#2476

The edit_distance API uses the Levenshtein algorithm.
This is used in NLP processes to compute the relative closeness between the characters in two words/strings.

The libcudf API looks like this:

std::unique_ptr<cudf::column> edit_distance(
  cudf::strings_column_view const& strings,
  cudf::strings_column_view const& targets,
  rmm::mr::device_memory_resource* mr)

And can be used to compute the edit distance between two columns of strings or a column of strings and a single string.

This PR will also include the appropriate Python/Cython interfaces to expose this through the cudf Python library.

[codecov]

Codecov Report

Merging #5811 into branch-0.15 will increase coverage by 0.32%.
The diff coverage is 87.50%.

@@               Coverage Diff               @@
##           branch-0.15    #5811      +/-   ##
===============================================
+ Coverage        84.11%   84.43%   +0.32%     
===============================================
  Files               80       80              
  Lines            13084    13411     +327     
===============================================
+ Hits             11006    11324     +318     
- Misses            2078     2087       +9     

Impacted Files	Coverage Δ
python/cudf/cudf/core/column/string.py	86.83% <87.50%> (+0.17%)	⬆️
python/cudf/cudf/io/feather.py	100.00% <0.00%> (ø)
python/cudf/cudf/comm/serialize.py	0.00% <0.00%> (ø)
python/custreamz/custreamz/_version.py	0.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/_version.py	0.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/io/tests/test_orc.py	100.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/io/tests/test_json.py	100.00% <0.00%> (ø)
...ython/dask_cudf/dask_cudf/io/tests/test_parquet.py	100.00% <0.00%> (ø)
python/cudf/cudf/utils/applyutils.py	98.75% <0.00%> (+0.02%)	⬆️
python/cudf/cudf/core/join/join.py	92.41% <0.00%> (+0.03%)	⬆️
... and 30 more

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[kkraus14]

Python / CMake LGTM!

[ramakrishnap-nv]

c++ LGTM