Add logic to save fair predictor to disk and load it back

[Innixma]

Users should be able to save their fair predictor to disk and load it back. Without this, they would need to fit a fair predictor every time they want to use one in a new python session, which isn't ideal since it also requires data to fit.

Note that in order to be portable, this needs to be implemented in a smart way, similar to how TabularPredictor's save/load logic works. This would add code complexity to do it right, but it is important for usability.

Sanity tests that should be done when implementing the logic:

• Test fitting fair predictor, saving to location A, then loading from location A, ensuring it works identically
• Test fitting fair predictor, saving to location A, manually moving to location B, then loading from location B, ensuring it works identically
• Test fitting fair predictor, saving to location A, manually moving to location B, then (in a new python session in a new working directory), loading from location B, ensuring it works identically