Use PDU not *Event in HeaderedEvent (#3073)

Requires https://github.com/matrix-org/gomatrixserverlib/pull/376

This has numerous upsides:
 - Less type casting to `*Event` is required.
- Making Dendrite work with `PDU` interfaces means we can swap out Event
impls more easily.
 - Tests which represent weird event shapes are easier to write.

Part of a series of refactors on GMSL.

userapi/consumers/roomserver.go

@@ -650,7 +650,7 @@ func (s *OutputRoomEventConsumer) evaluatePushRules(ctx context.Context, event *
 		roomSize: roomSize,
 	}
 	eval := pushrules.NewRuleSetEvaluator(ec, &ruleSets.Global)
-	rule, err := eval.MatchEvent(event.Event)
+	rule, err := eval.MatchEvent(event.PDU)
 	if err != nil {
 		return nil, err
 	}
@@ -698,7 +698,7 @@ func (rse *ruleSetEvalContext) HasPowerLevel(userID, levelKey string) (bool, err
 			continue
 		}
 
-		plc, err := gomatrixserverlib.NewPowerLevelContentFromEvent(ev.Event)
+		plc, err := gomatrixserverlib.NewPowerLevelContentFromEvent(ev.PDU)
 		if err != nil {
 			return false, err
 		}