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.

roomserver/internal/input/input_test.go

@@ -45,7 +45,7 @@ func TestSingleTransactionOnInput(t *testing.T) {
 		}
 		in := api.InputRoomEvent{
 			Kind:  api.KindOutlier, // don't panic if we generate an output event
-			Event: &types.HeaderedEvent{Event: event},
+			Event: &types.HeaderedEvent{PDU: event},
 		}
 
 		inputter := &input.Inputer{