[feature] Add custom fields to events

I would like to discuss the idea to give admins the option to add custom field to events. My idea would be: 1. Admins can define the name of the field and if the field is required or optional. 2. The field is shown on the "add event" page and depending on the settings is required to be filled before submitting an event 3. On the page of the event, the fields could be shown in a "key: value" like representation at a reasonable place on the page The motivation for this feature request is that for our instance we would like to have fields for "event organizer" and "accessibility", so that people submitting an event have to provide information on e.g. the name of their group or if the location/event is accessible with a wheelchair. We already ask people to provide this information, but often it is still missing. For our instance having the option to just add custom free-text field would be the easiest way I think. Regarding the "event organizer" field there was already some discussion in this issue: https://framagit.org/les/gancio/-/issues/362