Update FlexVotingClient so its interface is (more) aligned with the Governor

[apbendi]

The FlexVotingClient contract should use signatures that align with the Governor itself, i.e. treating the client almost like a mini-Governor. This is advantageous for a couple of reasons. For one, it is simply easier to reason about if you're already familiar with the Governor contracts and the way they work. Additionally, it allows existing tooling to be leveraged for clients with little-to-no changes, perhaps as far as allowing clients to be imported directly into tools like Tally so users could cast votes there. This effort would closely resemble the work done for L2 Flexible Voting to make the aggregator compatible with the Governor.