API to disable physics tick to run it manually

[Shatur]

Description

When developing a game with multiplayer, I need to run my physics steps exactly on network ticks. For example, libraries like naia defines their own Tick stage. The easiest way to synchronize them I need to disable physics step completely and add it to my networking Tick stage. I also will need to apply looping run criteria on physics (i.e. use ShouldRun::YesAndCheckAgain to run it multiply times in case of restoring from rollback).

Possible implementation

To achieve this I need two things:

1. Access to physics step system(s).
2. The ability to disable physics step.

For the first one we could just export systems which tick physics will allow user to add these systems to custom stages.
For the second one we could add a boolean option the plugin to disable physics steps scheduling.

Should be easy to implement, but I would appreciate to which systems I should disable and which one I should export to make it work correctly.

Going to play with this first locally to make sure it's enough for integration with networking.

[jcornaz]

I feel that there is an overlap, at least in user need, with #170 (related discussion: #168, implementation PR: #171)

I think we should consider the option of letting the user choose the stage. And if we go for a different solution, we'll have to re-assess the need of #170, as it's use-case is also rollback networking.

[Shatur]

Somehow I missed it all :D
Yes, this could solve my issue. I think I can replace looping run criteria with RollbackStage from naia side (need to request it), will be even better.