Notify users about Open Spaces schedule

Unlike programme sessions, Open Spaces are not announced by the bot — they live in Google Calendar and are invisible to Discord users.

It would be nice to notify users about Open Spaces via the room channels (e.g. `#s1`, `#s2`), similar to how programme sessions are announced. This would require a new data source integration since Open Spaces are not in the Programme API. We may consider adding Open Spaces to the Programme API to keep things consistent.

**Related:** [#373](https://github.com/EuroPython/discord/issues/373) — Post schedule updates (programme sessions)