Move account data types out of events

[jplatte]

Account data doesn't really have anything to do with events, even if the spec currently calls it that. See also https://github.com/matrix-org/matrix-doc/issues/3405.

[jplatte]

Maybe I'll do this as part of 0.5.0. It's an interesting work item. By moving these out, matrix-org/matrix-spec-proposals#686 also becomes a little more approachable I think.

[jplatte]

I'm a little torn on the naming here though. I think in terms of the SDK, it makes sense for these JSON objects to still be named "events" because they are also returned by /sync and you can subscribe to updates for them using client.register_event_handler right now.

[jplatte]

After attempting a clean split in #972 I found that that causes too much duplication. Also account data and other things the spec calls "events" still have in common that they're received in the sync response.

I'm still thinking it could be useful to have a separate feature flag / module structure for account data, but the derives will likely stay the same ones.