Accept boxed conditions for run_if, and cleanup IntoScheduleConfigs

[ItsDoot]

Objective

• Part of Extensible Schedule Graphs #20115
• Alternative to Create chainable run_if_dyn #18585

We can introduce another layer of traits in order to directly accept boxed conditions as the argument to IntoScheduleConfigs::run_if.

Solution

• Add IntoBoxedCondition trait which is used to convert IntoSystem conditions and BoxedConditon into BoxedConditions.
• Rename the ScheduleConfigs::x_inner functions to add_x/set_x and make them public. These are simply the &mut self versions of the functions on IntoScheduleConfigs, so we should expose them.
• Cleaned up the impl IntoScheduleConfigs for ScheduleConfigs since we now use those add_x functions in the default function implementations in IntoScheduleConfigs.

Testing

Added a new test for supplying a BoxedCondition to run_if.

[ecoskey]

Could this also be solved by implementing System for Box<dyn System>? Ran into a similar-ish thing here and I wonder if that might be a more direct fix than adding traits (and in my case extra methods/boxing). I know the maintainers have been iffy about that in the past though, so idk.

[ItsDoot]

Could this also be solved by implementing System for Box<dyn System>? Ran into a similar-ish thing here and I wonder if that might be a more direct fix than adding traits (and in my case extra methods/boxing). I know the maintainers have been iffy about that in the past though, so idk.

Yea cart has historically wanted to avoid doing that because of potential double/triple-boxing of systems. Note that you can already provide boxed systems via add_systems, this PR is just covering one of the remaining points: run_if.