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/internal/federationclient.go

@@ -27,7 +27,7 @@ func (a *FederationInternalAPI) MakeJoin(
 }
 
 func (a *FederationInternalAPI) SendJoin(
-	ctx context.Context, origin, s spec.ServerName, event *gomatrixserverlib.Event,
+	ctx context.Context, origin, s spec.ServerName, event gomatrixserverlib.PDU,
 ) (res gomatrixserverlib.SendJoinResponse, err error) {
 	ctx, cancel := context.WithTimeout(ctx, defaultTimeout)
 	defer cancel()