Make it possible for people to build a ‘local’ FB custom class

[xehu]

Some power users might want to use the infrastructure of the FeatureBuilder to build their own custom features/aggregations. In this case, it might be useful to allow users to create a "blank" FeatureBuilder class, within which they can incorporate user-defined functions. Especially since the FeatureBuilder pipeline is already fairly modular, this would leverage the existing modular design and open-source nature of the project and democratize it further.

This is also a great first step for making it easier for people to develop their own features, as is the case for scikit-learn:
https://scikit-learn.org/stable/developers/develop.html

While this is a long-range goal, this might be a great eventual extension to the toolkit.