Allow customizing serialization for feature generator extract functions

[tovbinm]

Related issues
It's not possible to save a model with a feature generator stage that has an extract function with ctor arguments.

Describe the proposed solution
Allow customizing read/write for extract functions, e.g.

// extract function IntMultiplyExtractor has an argument 'multiplier' passed during training time
val multiplier = 10
val multiplied = FeatureBuilder.Integral[Int].extract(new IntMultiplyExtractor(multiplier)).asPredictor

// one can now specify a read/write implementation to serialize for the extractor instance
@ReaderWriter(classOf[IntMultiplyExtractorReadWrite])
class IntMultiplyExtractor(val multiplier: Int) extends Function1[Int, Integral] with Serializable {
  def apply(i: Int): Integral = (i * multiplier).toIntegral
}
class IntMultiplyExtractorReadWrite extends ValueReaderWriter[IntMultiplyExtractor] {
  implicit val formats = DefaultFormats
  def read(valueClass: Class[IntMultiplyExtractor], json: JValue): Try[IntMultiplyExtractor] = Try {
    new IntMultiplyExtractor((json \ "multiplier").extract[Int])
  }
  def write(value: IntMultiplyExtractor): Try[JValue] = Try {
    "multiplier" -> JInt(value.multiplier)
  }
}

Describe alternatives you've considered
NA

[codecov]

Codecov Report

Merging #352 into master will increase coverage by 0.05%.
The diff coverage is 94.33%.

@@            Coverage Diff             @@
##           master     #352      +/-   ##
==========================================
+ Coverage   86.68%   86.73%   +0.05%     
==========================================
  Files         335      336       +1     
  Lines       10781    10799      +18     
  Branches      347      346       -1     
==========================================
+ Hits         9345     9367      +22     
+ Misses       1436     1432       -4

Impacted Files	Coverage Δ
...op/stages/DefaultOpPipelineStageReaderWriter.scala	69.23% <ø> (-0.77%)	⬇️
...m/salesforce/op/stages/FeatureGeneratorStage.scala	95.08% <100%> (+5.08%)	⬆️
...alesforce/op/stages/DefaultValueReaderWriter.scala	100% <100%> (ø)
...sforce/op/stages/OpPipelineStageReaderWriter.scala	85.71% <76.92%> (+1.09%)	⬆️
...la/com/salesforce/op/features/FeatureBuilder.scala	35.66% <0%> (+1.39%)	⬆️

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[michaelweilsalesforce]

Tested this internally. Seems to work as expected.