Give clients access to the room DAG

[jsparber]

Would be nice if it were possible to keep the local copy of the room timeline in sync with the server. Currently there is no way to know when new events were inserted in the timeline (e.g. when a federated server syncs at a later point). New events are included in sync but the correct placement in the event graph is unknown.

[dkasak]

@jsparber: Is "event_format": "federation" in filters what you want?

$ curl -d '{"event_format": "federation", "room": {"rooms": ["!GbZcxaXiuqXeGkhWQN:matrix.org"]}}' \
       -X POST "https://matrix-client.matrix.org/_matrix/client/v3/user/@dkasak_a:matrix.org/filter"
{"filter_id":"8"}

$ curl "https://matrix-client.matrix.org/_matrix/client/v3/sync?filter=8"

...

            {
              "auth_events": [
                "$kN_WAyr80wOIX5XiLjNQs-agOoAePjxFT90a49w1cQ0",
                "$Brmh-Z91ZQTAsLssg9BfP5PRJVRniQJCVvRJEdtpVQM",
                "$daBSHMf3srU1wJWjWwcXPjTNFUIO65j-Sc31aj0LElA"
              ],
              "content": {
                "body": "test",
                "msgtype": "m.text",
                "org.matrix.msc1767.text": "test"
              },
              "depth": 9,
              "hashes": {
                "sha256": "YxUiSgjJc+35qwqjWvCw94oROX0RmtyIaPX17eqqKaQ"
              },
              "origin": "matrix.org",
              "origin_server_ts": 1662480049004,
              "prev_events": [
                "$uV-G42JfjHrn0gYaCZaGvawYTYsJ0sBIFIUxLKsf9nU"
              ],
              "prev_state": [],
              "room_id": "!GbZcxaXiuqXeGkhWQN:matrix.org",
              "sender": "@dkasak_a:matrix.org",
              "type": "m.room.message",
              "signatures": {
                "matrix.org": {
                  "ed25519:a_RXGa": "YvmiQO/Ji9dxDNFem+CtCYdA910fHTyb5F9ehmfw3vGNmXiVL7FP6yOWGK/PQM7vCBTAFyEdtR9Zh2fMzdPYBQ"
                }
              },
              "unsigned": {
                "age": 324228,
                "transaction_id": "m1662480048867.2"
              },
              "event_id": "$5z2oYeg5UlSdtL3CPhuTaKrsQj-0A00ZE8vVXFBIOeQ"
            }

...