Improve digitize API

[isVoid]

Closes #6360

Updates digitize() API to accept any non-nullable column-like objects, as opposed to previously only np.array. Adds better error handling and further simplifies the implementation.

[codecov]

Codecov Report

Merging #7071 (91273b2) into branch-0.18 (6828e2c) will increase coverage by 0.01%.
The diff coverage is 80.00%.

@@               Coverage Diff               @@
##           branch-0.18    #7071      +/-   ##
===============================================
+ Coverage        82.09%   82.11%   +0.01%     
===============================================
  Files               97       97              
  Lines            16474    16479       +5     
===============================================
+ Hits             13524    13531       +7     
+ Misses            2950     2948       -2     

Impacted Files	Coverage Δ
python/cudf/cudf/core/column/numerical.py	94.17% <80.00%> (-0.24%)	⬇️
python/cudf/cudf/_fuzz_testing/fuzzer.py	0.00% <0.00%> (ø)
python/cudf/cudf/utils/hash_vocab_utils.py	100.00% <0.00%> (ø)
python/cudf/cudf/core/abc.py	91.48% <0.00%> (+4.25%)	⬆️
python/cudf/cudf/utils/gpu_utils.py	58.53% <0.00%> (+4.87%)	⬆️

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[shwina]

Approving with minor changes requested.