Change cudf::scalar copy and move constructors to protected

[davidwendt]

An instance of the cudf::scalar base class can currently through it's public copy constructor using the following example:

cudf::scalar b = cudf::numeric_scalar<int32_t>(8);

This line builds the numeric scalar instance, passes it to the base scalar's copy-ctor, and then destroys it.
(Simpler example here to show that this is happening: https://godbolt.org/z/hGEGvsqjq )
This means that the b instance is actually a plain cudf::scalar with no derived type.

Downcasting with dynamic_cast can runtime-check the object but we could have this line fail at compile time by making the cudf::scalar copy-ctor protected.

We could make the ctor explicit but I think making it protected would more clearly communicate the intention of the class.

This PR makes the cudf::scalar copy and move constructors protected to prevent instantiating a cudf::scalar at compile time.

[codecov]

Codecov Report

Merging #8857 (89a59b3) into branch-21.10 (18f7c01) will decrease coverage by 0.08%.
The diff coverage is n/a.

❗ Current head 89a59b3 differs from pull request most recent head ee30d95. Consider uploading reports for the commit ee30d95 to get more accurate results

@@               Coverage Diff                @@
##           branch-21.10    #8857      +/-   ##
================================================
- Coverage         10.67%   10.59%   -0.09%     
================================================
  Files               110      116       +6     
  Lines             18271    19034     +763     
================================================
+ Hits               1951     2017      +66     
- Misses            16320    17017     +697     

Impacted Files	Coverage Δ
python/cudf/cudf/__init__.py	0.00% <ø> (ø)
python/cudf/cudf/core/__init__.py	0.00% <ø> (ø)
python/cudf/cudf/core/column/categorical.py	0.00% <ø> (ø)
python/cudf/cudf/core/column/column.py	0.00% <ø> (ø)
python/cudf/cudf/core/column/lists.py	0.00% <ø> (ø)
python/cudf/cudf/core/column/numerical.py	0.00% <ø> (ø)
python/cudf/cudf/core/column/string.py	0.00% <ø> (ø)
python/cudf/cudf/core/column/struct.py	0.00% <ø> (ø)
python/cudf/cudf/core/dataframe.py	0.00% <ø> (ø)
python/cudf/cudf/core/frame.py	0.00% <ø> (ø)
... and 75 more

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[davidwendt]

@gpucibot merge