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.

federationapi/queue/queue_test.go

@@ -109,7 +109,7 @@ func mustCreatePDU(t *testing.T) *types.HeaderedEvent {
 	if err != nil {
 		t.Fatalf("failed to create event: %v", err)
 	}
-	return &types.HeaderedEvent{Event: ev}
+	return &types.HeaderedEvent{PDU: ev}
 }
 
 func mustCreateEDU(t *testing.T) *gomatrixserverlib.EDU {