Minimal core abstract framework to enable writing & reuse of custom gatekeepers. This milestone will replicate (almost) the same features as the MACI gatekeepers, but add reusability of criteria or state from other gatekeepers, separation of concerns between criteria definition, execution and enforcement, and more. By the end, anyone with some experience of gatekeepers/access control and Solidity development should be able to create, deploy and integrate a custom gatekeeper using the framework.