feat: allow #[prop(attrs)] on slots

[Upbolt]

Saw somebody in the Discord server ask for the ability to do

#[slot]
struct Slot {
  #[prop(attrs)]
  attributes: Vec<(&'static str, Attribute)>
}

Which currently is not implemented, so I just extended the existing options and slapped in the impls and methods in leptos_macro/slot.rs and leptos_macro/slot_helper.rs

[bram209]

didn't look into the implementation of prop spreading at all, so could be unrelated, but had an issue with prop spreading just now (not on slots):
#2403

leaving it here for reference, just in case

[benwis]

Some real serendipity here, thanks for contributing!