Readd - TimeStepMode to rapier configuration.

[Sirmadeira]

I opened this issue because I noticed that in 0.28 update, we lost TimeStepMode in RapierConfiguration. Since I am not sure ether this is supposed to happen or not I am just gonna leave it as an issue for now and them make a PR.
Nonetheless this will cause this warning all the time for those who use fixed schedule, and I have no idea on how to fix it.

Located here

[Vrixyz]

TimeStepMode is now an independent resource: https://docs.rs/bevy_rapier2d/0.28.0/bevy_rapier2d/plugin/enum.TimestepMode.html

[Sirmadeira]

Thing is, indepently on how I set it I keep getting the warning I guess the validation occurs. Before overriding timestep.

[Vrixyz]

The warning is emitted at the plugin build ; you can avoid it by setting your timestep resource before adding the physics plugin.

[Sirmadeira]

Or mid plugin built right? I guess I could do that.