SystemParamBuilder - Support buildable Vec parameters

[chescock]

Objective

Allow dynamic systems to take lists of system parameters whose length is not known at compile time.

This can be used for building a system that runs a script defined at runtime, where the script needs a variable number of query parameters. It can also be used for building a system that collects a list of plugins at runtime, and provides a parameter to each one.

This is most useful today with Vec<Query<FilteredEntityMut>>. It will be even more useful with Vec<DynSystemParam> if #14817 is merged, since the parameters in the list can then be of different types.

Solution

Implement SystemParam and SystemParamBuilder for Vec and ParamSet<Vec>.

Example

let system = (vec![
    QueryParamBuilder::new_box(|builder| {
        builder.with::<B>().without::<C>();
    }),
    QueryParamBuilder::new_box(|builder| {
        builder.with::<C>().without::<B>();
    }),
],)
    .build_state(&mut world)
    .build_system(|params: Vec<Query<&mut A>>| {
        let mut count: usize = 0;
        params
            .into_iter()
            .for_each(|mut query| count += query.iter_mut().count());
        count
    });

[alice-i-cecile]

I'm glad this is split out! The motivation in the PR description was very helpful, thanks :)

[alice-i-cecile]

@chescock let me know when merge conflicts are resolved and I'll get this in.