RFC: Update the established calendar Lexicon schema

[felinusfish]

Discussions moved to https://discourse.atprotocol.community/t/kicking-off-a-dedicated-space-for-discussion-for-calendar-events-rsvp/684/10?u=felinus.fish

With this RFC proposal, I am interested in making three impactful changes that will transform and reshape the lexicon of calendar.event and calendar.rsvp respectively. Ideally, with these changes, they bring much needed flexibility, along with a stronger definition in the process.

1. community.lexicon.calendar.event: Deprecate the "endsAt" property, to be eventually displaced by the use of "length" which should be defined as a positive value that begins at 0 (which would equate to "all day" for most clients), with an increase in the integer being reflected as startsAt + length as opposed to endsAt - startsAt, which has the benefits of being interchangeably flipped, along with a significantly easier stopgap in preventing events which end before startsAt is reached.

2. community.lexicon.calendar.event: Allow multiple String values in "mode" to allow for both "in-person" and "virtual" to be assigned to one event, as "hybrid" provides alternative connotations that do not reflect the use case of using both values at the same time. A "hybrid" event tends to refer to parts of an event being online, whilst others being in-person. Re-defining this property as an array would be beneficial long term.

3. community.lexicon.calendar.rsvp: Deprecate status string of "going," "not going" and "interested" to be superseded by a percentage value which can derive the original status values. 0% would replace "not going," 50% would replace "interested," and 100% would replace "going." By utilising a percentage, this allows for additional fine-grain control of how a user may want to represent the likelihood that they will attend the event, such as "90% chance of attendance" rather than "Interested" which does not provide enough valuable information for event arbitrators.

With regards to all of these changes, I have ensured that they are all within the realm of a positive impact, even with the consideration that they may impact the development of current and future event services, as these changes provide greater value to the Lexicon schema than what has been originally defined.