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

[nfrisby]

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 IntersectMBO/cardano-base#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.