[consensus/simplex] Add `Activity::ConflictingCertificate` Activity

[patrick-ogrady]

In some cases, we'll detect equivocation with ConflictingNotarize. However, a smart adversarial leader will send us only 1 notarize vote and they way we'll detect equivocation is by receiving a certificate for a proposal different than the leader's notarize. We can return this notarize or finalize and certificate as ConflictingCertificate (could be either notarization or finalization).