Include `event_id` when `/state/:type[/:key]?format=event`

[matrixbot]

This issue has been migrated from #15454.

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Problem

Currently when you GET /_matrix/client/v3/rooms/{room_id}/state/m.room.create?format=event (notice the event format), it doesn't include event_id which is kinda annoying to work with and reference.

Example:

{
    "content": {
        "creator": "@foo:bar",
        "room_version": "6"
    },
    "origin_server_ts": 1606978812896,
    "room_id": "!abc",
    "sender": "@foo:bar",
    "state_key": "",
    "type": "m.room.create",
    "unsigned": {
        "age_ts": 1606978812896
    }
}

Proposal

Add event_id to the output. There is no spec to go off of but seems like a non-breaking change we can do.

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Spawning from a need in matrix-org/matrix-viewer#167 to get the event_id of the m.room.create event in the room.

Is there a better way to do this? Perhaps /_matrix/client/v3/rooms/{room_id}/messages?dir=f&limit=1

Dev notes

• Relevant spec issue for the ?format query parameter: Spec ?format=event|content on GET /state/:type[/:key] matrix-org/matrix-spec#1047
• ?format introduced to Synapse in Allow clients to ask for the whole of a single state event matrix-org/synapse#1094

Relevant code:

https://github.com/matrix-org/synapse/blob/d935b806a52c967543f59def690aec0fa873d13c/synapse/rest/client/room.py#L258-L262

Currently, we use format_event_for_client_v2(data.get_dict()) against the EventBase PDU JSON dictionary which doesn't include event_id. We could instead use serialize_event(data, self.clock.time_msec()) which would get us something more workable.

We could also alternatively, add event_id to the dictionary returned by EventBase.get_dict() but that probably has too many downstream side-effects like messing with the PDU JSON serialization stuff.

https://github.com/matrix-org/synapse/blob/d935b806a52c967543f59def690aec0fa873d13c/synapse/events/__init__.py#L362-L366

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Separately, it would be good to standardize with the ?event_format=client|federation parameter used with other endpoints