Factory-like pattern for Hybrid Topology Factory (HTF) classes

[ijpulidos]

While working on https://github.com/choderalab/perses/pull/1050 it was noted that we can increase the readability and reduce some of the redundant code if we make a factory pattern for the hybrid topology factories objects, namely, HybridTopologyFactory and RESTCapableHybridTopologyFactory.

Specifically we can include a method to handle the creation of endstates and setting the different force parameters for them as noted in the comments in #1050

[ijpulidos]

One of the things we want to achieve here is that we probably want the HTF objects to have their own energy validation methods, more information in https://github.com/choderalab/perses/pull/1092#issuecomment-1226537892

[jchodera]

I'd like to see all the alchemical factories refactored to use a clean, well-designed API as part of the migration to openmmtools. The absolute alchemical factories can provide some inspiration for this.

A major goal would be to have base classes that support more facile experimentation with other approaches to nonbonded and valence alchemical modifications.

I'm not sure if OpenFE has any intentions to design APIs in this area, but I am presuming they do not at this point.