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.

internal/transactionrequest.go

@@ -184,7 +184,7 @@ func (t *TxnReq) ProcessTransaction(ctx context.Context) (*fclient.RespSend, *ut
 			t.rsAPI,
 			api.KindNew,
 			[]*rstypes.HeaderedEvent{
-				{Event: event},
+				{PDU: event},
 			},
 			t.Destination,
 			t.Origin,