[ML] Add new feature_processing field to Data frame analytics config

[benwtrent]

Data frame analytics jobs should handle a new configuration field called feature_processing.

This field will be an array of PreProcessor objects.

Requirements:

• feature_processing array gets serialized as a parameter to the analysis native process
• include/exclude works with custom fields.
  • excluded fields can STILL be used in a feature_processing processor.
  • a feature_processing processed field cannot be excluded.
• Before data is extracted and sent to the native process, the data needs to be pushed through the feature_processing processors

[elasticmachine]

Pinging @elastic/ml-core (:ml)