Merge pull request #1813 from matrix-org/erikj/make_membership_room_ver

MSC 1813 - Federation Make Membership Room Version

proposals/1812-federation-make-membership.md

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+# MSC 1813 - Federation Make Membership Room Version
+
+This proposal adds a new `room_version` field to the responses of `/make_leave`
+and `/make_join` APIs.
+
+## Motivation
+
+It is planned for future room versions to be able to change the format of events
+in various ways. To support this, all servers must know the room version
+whenever they need to parse an event.  Currently the `/make_*` APIs do not
+include the room version in the response, so the requesting server is unable to
+parse the event included in the response body.
+
+## Proposal
+
+Add a new `room_version` field to the response body of the `v1` `/make_join` and
+`/make_leave` APIs, which describes the room version.
+
+For backwards compatibility servers must correctly handle responses that do not
+include the new field. In which case the room version is assumed to be one of
+either `1` or `2`.