AutoML Pipeline – Integration to AutoML

[sebhrusen]

Sub-issue of #15854

AutoML must provide a mechanism that will activate the use of pipelines during the entire AutoML training, i.e. for base models training, for the HPO steps, and for the Stacked Ensemble models (note that SEs don't need to be encapsulated into Pipeline models, only their base models should be able to apply transformations when generating the metalearner's training frame, as well as when scoring).

** Requirements **
As a first integration example, replace the legacy/hacky integration of Target Encoding to AutoML with the new pipeline: this should allow HPO on some of the TE params during grid search.