Investigate ergonomic improvements for state scoped events API

[alice-i-cecile]

I have reverted back to clear_events_on_exit_state, but unlike with 4 it requires specifying the event type each time.

Quick question: if you use an impl Trait argument, rather than a generic, can we get type inference working properly for these methods?

Originally posted by @alice-i-cecile in #19435 (comment)