Enable responding with canonical JSON for federation.

[jevolk]

Due to the conclusions of matrix-doc matrix-org/matrix-spec#255 the server should only transmit the exact string of bytes that it signed (canonical JSON). This probably reverts matrix-org/matrix-spec-proposals#878.

Please consider this high priority. The change is trivial and can be shipped immediately.

[reivilibre]

This one is quite an old issue, but my reading of it is that sending out canonical JSON over the wire allows the receiving homeserver to be able to optimistically support checking the signature of the event, without need to first reserialise the event as Canonical JSON (except there would be a few catches due to some fields needing to be skipped).

There have also been suggestions about just using CBOR or friends to reduce the faff, but I suppose none of that materialised since 2018 :-).