Add relationships between events

[ThisIsMissEm]

For conferences, it makes sense to have one event for the conference as a whole, and then the ability to declare a parent event relationship for the individual sessions. Currently this is not possible to represent natively within the community lexicon.

cc @flo-bit

[flo-bit]

Yeah I wrote down some notes in the discourse already, agreed on the parentEvent, I will probably add some other (optional) fields at least for events created with atmo.rsvp and for most of those it would probably be nice to add that to the actual community lexicon too.

Main ones I'm currently thinking:

• bskyPostRef (e.g. for comments)
• parentEvent
• hosts (if someone creates an event for someone else or multiple people host an event, not sure how this would work re accepting being host)
• calendarRef (for luma like calendars where multiple people can add events to a "shared" calendar)