Timestamp-based Governor

[ericnordelo]

🧐 Motivation

I'm using your Governance implementation, and I want to check proposal deadlines with Timestamps instead of Block Numbers.

Currently, you have a hardcoded Timer.BlockNumber in the ProposalCore struct inside the Governor contract, and I made some research through your implementation checking how to change this behavior. In this search I found that you have already a Timer for timestamps with the same interface as block numbers in the Timers library, so changing the ProposalCore struct to Timers.Timestamp achieves what I want.

The issue is that in order to change this struct, right now I would need to define another Governor contract and fix the inheritance tree (to redefine the struct), or override a lot of functions to fix the usage of ProposalCore with a new one (struct), I think that is too complicated for the purpose of the library.

So I propose to use a Generic Timer inside the Timer library that allows us to define the behavior depending on some configurations, specifically in a virtual method in Governor called timerType (added to the interface to get the current Timer used on the Governance), which can be easily overridden to select what Timer we want to use, and we could add also a config in the GovernorSettings module to allow swapping between this types in production. I made a PR for this and is linked to this issue.

[ericnordelo]

The PR to be reviewed:

#3080