Expose information about system-internal parallelism to the scheduler

[alice-i-cecile]

What problem does this solve or what need does it fill?

Whether or not a system uses parallelism, such as via Query::par_iter, is extremely important information for optimizing performance.

Multiple heavily parallel systems should not be run beside each other at the same time.

Possible solutions

1. Manually mark a system as parallel via system config, as a type of performance hint for schedulers.
2. Require a special ParQuery system param to access Query::par_iter
3. Have a virtual unit type resource ParallelAccess, and require the user pass in &mut ParallelAccess into Query::par_iter to get it to compile.

I think that 3 is simply a better form of 2. We may want scheduler performance hints in some form anyways though, even if we don't end up using it for parallelism.

Additional context

Prompted by @ItsDoot: https://discord.com/channels/691052431525675048/749335865876021248/1385332129269022790