Add encryption to the suggested invite_state

[turt2live]

https://matrix.org/docs/spec/server_server/r0.1.4#put-matrix-federation-v2-invite-roomid-eventid

[turt2live]

This happened somewhere, I think. If not, it's a small MSC that doesn't need a tracking issue.

[clokep]

MSC3173 specced this.