The aim of this ticket is to prototype separating feature-specific parts of the upgrade/downgrade process separately from the core setFCV implementation (essentially, a plugin system for setFCV):

• For each feature, we can define a C++ class that implements one or more functions (e.g. upgradeMetadata, downgradeMetadata, etc.).
• Those C++ classes can register themselves as steps that should be called as part of the FCV upgrade/downgrade done by setFCV.
• From setFCV, we should be able to each registered step from the existing placeholders (_upgradeServerMetadata, _prepareToUpgradeActions, etc.).
• It should play well with the code owners system (adding a new step should not be forced to be reviewed by a Server team other than 10gen/server-fcv).

We should find if there is some system that is built in a similar way that we can take as a reference.

Ideally, once the API is well defined we can take turn the proof of concept into commitable code.