Name Prefix on FeatureSpec

[richwhitjr]

It would be nice to be able to have a function on FeatureSpec that allows for a prefix on each Transformer name.

FeatureSpec.of[T].withNamedPrefix("my_prefix").required(...)

The main reason is that a common pattern recently to have many FeatureSpecs that you may want to drop or include in a job. It is nice to have a prefix on the names so later it is possible to track down which group of features this particular feature originated.

The tough part of implementing it is that internally the FeatureSpec only has a list of Transformers where name is a val. So it is not possible to add a prefix since a transformer can not be copied and the name is not mutable.