Allow configuring fullscreen material system order

[beicause]

Objective

Fixes #23781

When Hdr is on, the order of tonemapping and fullscreen_material_system is uncertain and the result of main pass can be cleared. run_in run_before run_after can only specify system set and can't specify a specific system and them hardcode Core3dSystems.

Solution

Replace run_in run_before run_after with fn schedule_configs(system: ScheduleConfigs<BoxedSystem>) -> ScheduleConfigs<BoxedSystem>

Testing

Run fullscreen_material example with Hdr

[IceSentry]

I did not know this ScheduleConfigs api existed, that's a much nicer solution.

Can you update the comment in the example that shows how to set the config for 2d?