Support setting to a new column in DataFrame.loc

[isVoid]

closes #7628

This PR adds support to setting a column in the dataframe when the provided column name is a new column name. The specified rows can be of a single row label, a collection of row labels, or slices. The value-to-set can be column-like object or scalar. E.g. you can now do this:

>>> x = cudf.DataFrame()
>>> x.loc[:, "a"] = [1, 2, 3] # set a new column with list
>>> x
   a
0  1
1  2
2  3
>>> x.loc[[1, 2], "b"] = ["abc", "cba"] # set part of the new column with list
>>> x
   a     b
0  1  <NA>
1  2   abc
2  3   cba
>>> x.loc[:, "c"] = 5 # set the new column to the scalar
>>> x
   a     b  c
0  1  <NA>  5
1  2   abc  5
2  3   cba  5

[galipremsagar]

rerun tests

[codecov]

Codecov Report

Merging #8012 (5537aa7) into branch-0.20 (51336df) will increase coverage by 0.04%.
The diff coverage is 90.47%.

❗ Current head 5537aa7 differs from pull request most recent head 148f0f3. Consider uploading reports for the commit 148f0f3 to get more accurate results

@@               Coverage Diff               @@
##           branch-0.20    #8012      +/-   ##
===============================================
+ Coverage        82.88%   82.93%   +0.04%     
===============================================
  Files              103      103              
  Lines            17668    17678      +10     
===============================================
+ Hits             14645    14662      +17     
+ Misses            3023     3016       -7     

Impacted Files	Coverage Δ
python/cudf/cudf/core/column/__init__.py	100.00% <ø> (ø)
python/cudf/cudf/io/orc.py	86.89% <ø> (ø)
python/cudf/cudf/utils/cudautils.py	57.75% <25.00%> (ø)
python/cudf/cudf/core/column/column.py	88.64% <71.42%> (ø)
python/cudf/cudf/core/column/numerical.py	94.43% <72.72%> (ø)
python/dask_cudf/dask_cudf/backends.py	89.51% <80.00%> (-0.08%)	⬇️
python/cudf/cudf/core/dataframe.py	90.87% <83.33%> (+0.01%)	⬆️
python/cudf/cudf/utils/utils.py	89.53% <91.66%> (+0.02%)	⬆️
python/cudf/cudf/core/column/datetime.py	89.91% <100.00%> (ø)
python/cudf/cudf/core/column/timedelta.py	88.66% <100.00%> (ø)
... and 21 more

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[kkraus14]

@gpucibot merge