Remove free function aggregation with feature builders

[tovbinm]

Related issues
Allowing arbitrary function aggregation prevents us from implementing serialization for FeatureGeneratorStage instances required in #274

Describe the proposed solution
Remove free function aggregation with feature builders

Describe alternatives you've considered
NA

Additional context
Users can still add custom aggregators using objects of classes as follows:

val survivedResponse = FeatureBuilder.Binary[Passenger]
    .extract(_.survived.toBinary)
    .aggregate(MyAggr)
    .asResponse

object MyAggr extends CustomMonoidAggregator[Binary](zero = ..., associativeFn = ...)
// or
class MyAggr() extends CustomMonoidAggregator[Binary](zero = ..., associativeFn = ...)

the old syntax wont work anymore:

// not supported anymore
val survivedResponse = FeatureBuilder.Binary[Passenger]
    .extract(_.survived.toBinary)
    .aggregate(zero = ..., associativeFn = ...)
    .asResponse

// not supported anymore
val survivedResponse = FeatureBuilder.Binary[Passenger]
    .extract(_.survived.toBinary)
    .aggregate(associativeFn = ...)
    .asResponse

[wsuchy]

LGTM

[codecov]

Codecov Report

Merging #311 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #311      +/-   ##
=========================================
+ Coverage   86.48%   86.5%   +0.01%     
=========================================
  Files         322     322              
  Lines       10583   10579       -4     
  Branches      347     564     +217     
=========================================
- Hits         9153    9151       -2     
+ Misses       1430    1428       -2

Impacted Files	Coverage Δ
...sforce/op/aggregators/CustomMonoidAggregator.scala	100% <ø> (ø)	⬆️
...la/com/salesforce/op/features/FeatureBuilder.scala	32.85% <ø> (-1.41%)	⬇️
...com/salesforce/op/test/PassengerFeaturesTest.scala	100% <100%> (ø)	⬆️
...es/src/main/scala/com/salesforce/op/OpParams.scala	89.79% <0%> (+4.08%)	⬆️

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