Specify the /invite federation endpoint

[babolivier]

This endpoint (/_matrix/federation/v1/invite/{roomID}/{eventID}) is to be requested using the PUT method and its body must contain an invite m.room.member event.

It is requested in the case a homeserver A wants to invite a user located on a homeserver B on a room located on A. A will then build a m.room.member event and send it to B so B can append its signature to the event and respond with the updated event to acknowledge it.

Due to a typo in Synapse kept for ensuring compatibility, the response body must be constructed as:

200 OK

[
     200,
    {
        "event": {...}
    }
]

with the updated event at the event key.