Add length of the text as default features for text fields

[tmpsrcrepo]

Related issues
Add text length as a new feature

Describe the proposed solution
Created TextLenTransformer and TextMapLenEstimator and modify the vectorizer to include a new toggle feature

Describe alternatives you've considered

[codecov]

Codecov Report

Merging #187 into master will decrease coverage by 0.15%.
The diff coverage is 83.87%.

@@            Coverage Diff             @@
##           master     #187      +/-   ##
==========================================
- Coverage   86.31%   86.15%   -0.16%     
==========================================
  Files         307      301       -6     
  Lines        9971     9819     -152     
  Branches      336      345       +9     
==========================================
- Hits         8606     8460     -146     
+ Misses       1365     1359       -6

Impacted Files	Coverage Δ
.../com/salesforce/op/features/TransientFeature.scala	64.28% <0%> (-6.31%)	⬇️
...e/op/stages/impl/feature/TextMapLenEstimator.scala	100% <100%> (ø)
...sforce/op/utils/spark/OpVectorColumnMetadata.scala	77.77% <100%> (+2.22%)	⬆️
...p/stages/impl/feature/SmartTextMapVectorizer.scala	100% <100%> (ø)	⬆️
...ce/op/stages/impl/feature/TextLenTransformer.scala	100% <100%> (ø)
...e/op/stages/impl/feature/SmartTextVectorizer.scala	97.72% <100%> (+0.32%)	⬆️
...c/main/scala/com/salesforce/op/ModelInsights.scala	92.56% <100%> (-0.07%)	⬇️
...n/scala/com/salesforce/op/dsl/RichMapFeature.scala	79.1% <30%> (-2.44%)	⬇️
.../scala/com/salesforce/op/dsl/RichTextFeature.scala	89.23% <71.42%> (-1.1%)	⬇️
...sforce/op/stages/impl/feature/Transmogrifier.scala	95.49% <78.26%> (-1.26%)	⬇️
... and 66 more

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[tovbinm]

This PR was split into #190, #195 and merged in.