Idea: Allow ValidatorManager to be deployed as a standalone contract

[cam-schultz]

Context and scope
While our implementation uses an inheritance tree to resolve to a single deployed contract, there is demand for multiple contract designs, in which the base ValidatorManager tracks the validator set, and dispatches other logic to specialized contracts. For example, the proposed ACP-99 outlines an architecture consisting of a ValidatorManager and a SecurityModule, who hold references to each other's deployed addresses. There is an active discussion to provide a unified interface for validator managers that is agnostic to architecture.

Discussion and alternatives
This issue is to experiment with extensions to ValidatorManager that would allow it to be deployed in a standalone manner such that it dispatches specialized logic to external contracts. For example, a variant of the two-contract SecurityModule design is implemented here. That PR, however, is very much coupled with a two-contract design, so is not particularly general.

This issue should explore frameworks that allow the logic in ValidatorManager to be easily adapted to any architecture.