Refine interface for node creating `BlockForging` closures for the sake of KES Agent

The existing Consensus interface used by the node code to create `BlockForging` structures (eg upon init and SIGHUP) that are then passed to Consensus does not obviously suffice for the needs of the https://github.com/IntersectMBO/cardano-base/issues/312 (owned by @tdammers) refinement. This Issue is to create an alternative path that is more obviously compatible with the KES Agent's needs, and to establish confidence (eg via tests) that it is otherwise equivalent in behavior to the existing path.