`IntegrationParameters` is inconvenient to configure at startup due to private fields on `RapierContext`

[Jondolf]

I was trying to configure the number of solver iterations, which reside in IntegrationParameters, stored in the RapierContext resource. Like how most resources in Bevy are typically configured at start-up, I tried inserting the resource:

fn main() {
    App::new()
        .add_plugins((DefaultPlugins, RapierPhysicsPlugin::<()>::default()))
        .insert_resource(RapierContext {
            integration_parameters: IntegrationParameters {
                num_solver_iterations: NonZeroUsize::new(6).unwrap(),
                ..default()
            },
            ..default() // error: "field `event_handler` of struct `bevy_rapier2d::plugin::RapierContext` is private" and so on
        })
        .run();
}

The property spread operation fails, because there are several private fields. There also doesn't seem to be a way to configure the integration parameters directly through a constructor or builder method.

Instead, an intermediary variable needs to be created for the RapierContext.

fn main() {
    let mut ctx = RapierContext::default();
    ctx.integration_parameters = IntegrationParameters {
        num_solver_iterations: NonZeroUsize::new(6).unwrap(),
        ..default()
    };

    App::new()
        .add_plugins((DefaultPlugins, RapierPhysicsPlugin::<()>::default()))
        .insert_resource(ctx)
        .run();
}

This isn't immediately obvious, and it's different from how resources are typically configured.

Compare this to changing the substep count in e.g. bevy_xpbd:

fn main() {
    App::new()
        .add_plugins((DefaultPlugins, PhysicsPlugins::default()))
        .insert_resource(SubstepCount(6))
        .run();
}

Either the fields on RapierContext should be made public, or (preferably) the IntegrationParameters should be extracted into their own resource.