Feature munging pipeline for AutoML

[sebhrusen]

As AutoML wants to apply transformations to the features from the original dataset, we need to encapsulate those transformations in a Pipeline model that can then be returned to the user to ensure that those same transformations are also applied for predictions.

As a parent issue, this is divided into the following tasks:

• [x] https://github.com/h2oai/h2o-3/issues/15855
• [x] https://github.com/h2oai/h2o-3/issues/15856
• [x] https://github.com/h2oai/h2o-3/issues/15857
• [ ] https://github.com/h2oai/h2o-3/issues/15859

Independent issues raised for proper implementation of pipeline tasks:

• [x] https://github.com/h2oai/h2o-3/issues/15858

Optional/future improvements

• [ ] Mojo support.
• [ ] Provide client API to use those pipelines with any kind of H2O-3 model.