Add ffill and bfill to string columns

[isVoid]

Follow up of PR #7004

Adds method field to fillna method in string type column to support ffill and bfill.
Also involves a small change to a datetime64 ffill, bfill test case to improve test robustness.

[codecov]

Codecov Report

Merging #7036 (9b24a4e) into branch-0.18 (4385f54) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           branch-0.18    #7036      +/-   ##
===============================================
+ Coverage        82.05%   82.07%   +0.01%     
===============================================
  Files               96       96              
  Lines            16406    16415       +9     
===============================================
+ Hits             13462    13472      +10     
+ Misses            2944     2943       -1     

Impacted Files	Coverage Δ
python/cudf/cudf/core/column/string.py	86.62% <100.00%> (+0.13%)	⬆️
python/cudf/cudf/core/frame.py	89.97% <0.00%> (-0.29%)	⬇️
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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