Enable Custom Estimators in Entrypoint

[f-allian]

• Our main entry-point currently only supports LinearRegression and LogisticRegression estimators by directly hardcoding them in.
• We should enable a factory/registry to support custom estimator classes that can be programmatically registered and reference by name in test configuration files.
• This is important as we have already have the base abstract estimator classes, and some examples in the examples directory rely on custom classes.