Add support for effects with custom runtimes

[LLBlumire]

Fixes #3468

This is almost certainly not the best way to do it, but I couldn't figure out the generic types so did a pinned box instead, but it demonstrates the functionality that is desired.

[gbj]

This looks good to me, overall! Couple small notes on docs, and a suggestion if you do want to use generics rather requiring the function to accept a pinned-box -- although you should probably make that decision based on how you're going to consume it! (i.e., if the current signature is easier to use then go with this one)

[gbj]

Docs should probably be updated to

1. Note that this allows you to pass in a runtime, and
2. Update the new_sync x 2 here to new_sync_with_runtime

[gbj]

I'd suggest a name like spawn here rather than pass_to_rt as that's the phrase we tend to use for "spawn a task" elsewhere

I think the signature can be something like

    pub fn new_with_runtime<T, M, F, Fut>(
        mut fun: impl EffectFunction<T, M> + 'static,
        pass_to_rt: F
    ) -> Self 
    where 
        T: 'static,
        F: FnOnce(Fut) + 'static,
        Fut: Future<Output = ()>

if you want to avoid the Pin<Box<_>> here