Forces read only (#908)

# Objective

Fixes #907

## Solution

This PR splits the `RigidBodyForces` trait into `ReadRigidbodyForces` and `WriteRigidBodyForces`, and implements `ReadRigidBodyForces` for the `ForcesReadOnlyItem` type. This allows for immutable functions to be called on a `Forces` item immutably retrieved from queries, as shown below.

## Testing

Tested by building the example shown in the issue:

```rust
fn get_total_inertia(objects: Query<(Forces, &Mass)>) {
    let total = objects
        .iter()
        .map(|(forces, mass)| forces.linear_velocity() * mass.0)
        .reduce(|a, b| a + b);
}
```
Which now compiles without errors and works as expected.

Author: Exotik850

migration-guides/0.4-to-main.md

@@ -5,3 +5,13 @@ since the latest release. These guides are evolving and may not be polished yet.
 
 See [migration-guides/README.md](./README.md) and existing entries for information about Avian's
 migration guide process and what to put here.
+
+## `ReadRigidBody` and `WriteRigidBody`
+
+PR [#908](https://github.com/avianphysics/avian/pull/908) introduced two new traits: `ReadRigidBody` and `WriteRigidBody`, and `RigidyBodyForces` is now defined as:
+
+```rust
+pub trait RigidBodyForces: ReadRigidBodyForces + WriteRigidBodyForces {}
+```
+
+In most cases this should just work, but if it doesn't, you can replace your implementation for `RigidBodyForces` with both `ReadRigidBodyForces` and `WriteRigidBodyForces` where it is used / needed. Both traits are required to implement `RigidBodyForces`, but you can implement them separately.

src/dynamics/mod.rs

@@ -87,7 +87,7 @@ pub mod prelude {
             forces::{
                 ConstantAngularAcceleration, ConstantForce, ConstantLinearAcceleration,
                 ConstantLocalForce, ConstantLocalLinearAcceleration, ConstantTorque, ForcePlugin,
-                ForceSystems, Forces, RigidBodyForces,
+                ForceSystems, Forces, ReadRigidBodyForces, RigidBodyForces, WriteRigidBodyForces,
             },
             mass_properties::{
                 MassPropertiesExt, MassPropertyHelper, MassPropertyPlugin,

src/dynamics/rigid_body/forces/mod.rs

@@ -201,7 +201,10 @@ mod query_data;
 mod tests;
 
 pub use plugin::{ForcePlugin, ForceSystems};
-pub use query_data::{Forces, ForcesItem, NonWakingForcesItem, RigidBodyForces};
+pub use query_data::{
+    Forces, ForcesItem, NonWakingForcesItem, ReadRigidBodyForces, RigidBodyForces,
+    WriteRigidBodyForces,
+};
 
 use crate::prelude::*;
 use bevy::prelude::*;