This patch updates `sync::sync_events::v4::Request` to add the new
`bump_event_types` field, as described in [MSC3575] (more precisely in
this [diff]).
That's the first step to address https://github.com/matrix-org/matrix-rust-sdk/issues/1728.
[MSC3575]: https://github.com/matrix-org/matrix-spec-proposals/blob/kegan/sync-v3/proposals/3575-sync.md
[diff]: 35b79f6ebe...5bd13e66df (diff-1f7276cbbee4eeaaca06b07d2fe58312eab735259f2490e28e04710dde77fdccL78)