Introduce a new stream for the appservice consumer (#3277)

This introduces a new stream the syncAPI produces to once it processed a
`OutputRoomEvent` and the appservices consumes.
This is to work around a race condition where appservices receive an
event before the syncAPI has handled it, this can result in e.g. calls
to `/joined_members` returning a wrong membership list.

syncapi/producers/appservices.go

@@ -0,0 +1,33 @@
+// Copyright 2023 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package producers
+
+import (
+	"github.com/nats-io/nats.go"
+)
+
+// AppserviceEventProducer produces events for the appservice API to consume
+type AppserviceEventProducer struct {
+	Topic     string
+	JetStream nats.JetStreamContext
+}
+
+func (a *AppserviceEventProducer) ProduceRoomEvents(
+	msg *nats.Msg,
+) error {
+	msg.Subject = a.Topic
+	_, err := a.JetStream.PublishMsg(msg)
+	return err
+}