To-device messages have an unspecified, required `message_id` field

[babolivier]

#9686 introduced a message_id field in to-device messages for tracing purposes, which isn't in the spec, and which Synapse sends to all to-device messages it sends. This is confusing because the name does not imply it's intended to live out of the spec.

In an even more problematic way, Synapse also mandates it for incoming to-device EDUs (ref), meaning that other server implementations have to also implement it.

Also see discussion in #matrix-spec: https://matrix.to/#/!NasysSDfxKxZBzJJoE:matrix.org/$PWq5IoB9WUfKdUiyRjTOUG5tCFNT6pBcBT8uwUcQVTg?via=matrix.org&via=libera.chat&via=element.io

[babolivier]

It is actually in the spec. Ignore me.