Implement Save/Load using pickle

Current save/load methods focus on dumping and loading the pipeline definition in its JSON form, but provide no means to save a fitted pipeline and load it later to make predictions, being the usage of pickle outside of the pipeline the only way to go.

Let's re-implement the save/load methods to save the whole pipeline instance, and move the current save functionality to a to_json method.

Note that from_json is not needed anymore because the pipeline annotation can already be passed as a JSON path in the pipeline __init__ method.

Start by locating the pipeline save/load methods and the pipeline init handling of JSON paths. Review how the current JSON definition is serialized, then verify that save/load preserve a fitted pipeline for later predictions while the JSON export remains available as to_json. No test files are named in the issue, so identify the relevant serialization or pipeline tests before changing behavior.