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_test.go

@@ -636,7 +636,7 @@ func init() {
 		if err != nil {
 			panic("cannot load test data: " + err.Error())
 		}
-		h := &rstypes.HeaderedEvent{Event: e}
+		h := &rstypes.HeaderedEvent{PDU: e}
 		testEvents = append(testEvents, h)
 		if e.StateKey() != nil {
 			testStateEvents[gomatrixserverlib.StateKeyTuple{