Clarify how or make it easier to pass in collections that are T: Iterator<Item=D> in builder functions for helix requests

[Emilgardis]

This came up in discussion over at Twitch API Discord

currently, it can be hard to reason how to deal with a method like

fn(Self, impl Into<Cow<'a, [&'a Borrowed]>>)

we have a bunch of these after #280

One way to deal with this, given we have a Vec<String>, is

fn builder(s: impl Into<Cow<'a, [&'a Borrowed]>>) -> Foo {
    todo!()
}

fn bar(items: Vec<String>) {
    let items: Vec<&Borrowed> = items.iter().map(Into::into).collect();
    builder(items)
}

but coming up with this is not apparent.

This is related to #114 (comment) where we would make the allocation optional

[Emilgardis]

We could clarify in the documentation for the methods how to do this, or make new methods that take impl Iterator<Item = impl Into<&'a Borrowed>> which are then collected and put in a Cow::Owned.

I think I prefer the second alternative, since one would have to do an allocation either way if you have a Vec<Owned>.

@laundmo also mentions in the discord thread a solution that would involve a impl CollectionRef which abstracts over a collection, this would work for the impl Iterator route, and it would also work for a route where we make the stored collection in the Request generic over collection for those fields, albeit at a cost of more generics on that specific request.

[laundmo]

heres my CollectionRef idea:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=14019aa8f991b7ec5eef82f29879a459

a great advantage is that this works on many collections and even individual items

[Emilgardis]

This could also be applied for the client ext methods which currently takes impl AsRef<[&'client types::CategoryIdRef]>

[Emilgardis]

also experimented with

fn into_slice_cow(self) -> std::borrow::Cow<'a, [R]>
    where
        [R]: ToOwned,
        R: Sized + 'a, {
        match self {
            std::borrow::Cow::Borrowed(b) => {
                std::borrow::Cow::Borrowed(std::slice::from_ref(b.into()))
            }
            std::borrow::Cow::Owned(o) => {
                let a: std::borrow::Cow<'_, R> = std::borrow::Cow::Owned(o.into());

                let a = std::slice::from_ref(a.as_ref());

                std::borrow::Cow::Borrowed(a) // ~ cannot return value referencing local variable `a`
            }
        }
    }