derive(Event) option to use single buffering

[ItsDoot]

What problem does this solve or what need does it fill?

For some types of events, like an UpdatePosition event, data that is one frame/tick behind might be useless. Having the option to disable double buffering and use single buffering per event type could be a nice performance improvement.

What solution would you like?

#[derive(Event)]
#[event(buffer = "single")]
struct PositionEvent {
    x: i32,
    y: i32,
    z: i32,
}

What alternative(s) have you considered?

Continue to use double buffering.

[ItsDoot]

Related issue: #2071
Related PRs: #7957 and #2073

Note: this issue, unlike #2071, is specifically about the buffering aspect, not the data type used to store them.

[alice-i-cecile]

I would also really like to see buffer = "manual", ideally as part of the same PR.