Add a system's entity to run conditions, when we have them

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

This is a bit early, but once #16618 lands, information about a system will be found by using it's entity, something that might be useful for run conditions to have.

## What solution would you like?

```rust
#[derive(Component)]
pub struct RunComponent;

fn run_if_has_component(system: Entity, run_components: Query<(), With>) -> bool {
run_components.contains(system)
}
```

Obviously this example isn't representative of what people would be doing, it's mostly just the simplest example I could think of.

## What alternative(s) have you considered?

Use the current method.

## Additional context

This may also be able to help with #19712

Start by reviewing issue #16618 and the current run-condition API to understand how a system entity could become available. Then consider the relationship to #19712; the work is complete only once the intended API and behavior for exposing that entity to run conditions are agreed and implemented.

Written by the indexing model from the issue text.