[Discussion] Refactor worker class

It is ok to inject those feature engineering procedures into an FL course in this way, but we shall change the instantiation of workers to a better (more general and unified) way. Actually, it is quite confusing to wrap a worker by feature engineering-related wrapper. Feature engineering is just a tiny step in an FL course, which doesn't change a worker significantly. One usual way to instantiate a worker from a collection of such pluggable behaviors is to use factory pattern I guess.

_Originally posted by @joneswong in https://github.com/alibaba/FederatedScope/pull/426#discussion_r1046631649_

No files, tests, or entry points are identified. Start by locating worker instantiation and the feature-engineering-related wrapper, then determine how a general factory could unify pluggable worker behaviors. Done means workers can be instantiated through the agreed general approach without confusing feature-engineering wrappers.

Written by the indexing model from the issue text.