Scrubbing task: rest of transforms

[artidoro]

Fixes: #2835.

This PR does the scrubbing for the following transforms:

• ReplaceMissingValues
• IndicateMissingValues
• CustomMapping

[codecov]

Codecov Report

Merging #2876 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2876      +/-   ##
==========================================
+ Coverage   71.69%   71.71%   +0.02%     
==========================================
  Files         811      812       +1     
  Lines      142546   142676     +130     
  Branches    16125    16124       -1     
==========================================
+ Hits       102193   102321     +128     
- Misses      35924    35941      +17     
+ Partials     4429     4414      -15

Flag	Coverage Δ
#Debug	71.71% <100%> (+0.02%)	⬆️
#production	67.89% <100%> (-0.02%)	⬇️
#test	85.99% <100%> (+0.08%)	⬆️

Impacted Files	Coverage Δ
...rc/Microsoft.ML.Transforms/CustomMappingCatalog.cs	100% <ø> (ø)	⬆️
src/Microsoft.ML.Transforms/ExtensionsCatalog.cs	75% <100%> (ø)	⬆️
....ML.Transforms/MissingValueIndicatorTransformer.cs	69.66% <100%> (ø)	⬆️
...s/Api/CookbookSamples/CookbookSamplesDynamicApi.cs	80.4% <100%> (ø)	⬆️
src/Microsoft.ML.FastTree/RegressionTree.cs	71.42% <0%> (-12.25%)	⬇️
...oft.ML.Data/Dirty/ChooseColumnsByIndexTransform.cs	91.85% <0%> (-5.93%)	⬇️
...c/Microsoft.ML.SamplesUtils/SamplesDatasetUtils.cs	18.88% <0%> (-4.85%)	⬇️
src/Microsoft.ML.Data/Transforms/RangeFilter.cs	69.51% <0%> (-2.85%)	⬇️
...rc/Microsoft.ML.Data/Scorers/RowToRowScorerBase.cs	88.06% <0%> (-2.47%)	⬇️
...t.ML.Data/Transforms/ValueToKeyMappingEstimator.cs	85.18% <0%> (-1.61%)	⬇️
... and 39 more

[rogancarr]

        public bool IsUnderThirty { get; set; }

Move to first entry (because it's a label) or alphabetize the list. #Resolved

---

Refers to: docs/samples/Microsoft.ML.Samples/Dynamic/Transforms/CustomMappingSample.cs:58 in 0f5ecdc. [](commit_id = 0f5ecdc, deletion_comment = False)

[rogancarr]

Just a few nits, then it's ready for the limelight.
👨‍🚀 🛰 👾

[Ivanidzo4ka]