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/storage/postgres/state_snapshot_table.go

@@ -242,7 +242,7 @@ func (s *stateSnapshotStatements) BulkSelectMembershipForHistoryVisibility(
 			// not fatal
 			continue
 		}
-		he := &types.HeaderedEvent{Event: event}
+		he := &types.HeaderedEvent{PDU: event}
 		result[eventID] = he
 		knownEvents[membershipEventID] = he
 	}