Add ColliderHandle:s to PairFilterContext

This makes it easy to write custom filters based on the handles.

For instance, a user can add pairs of `ColliderHandle`s into a `HashSet` and
do a lookup in a `ContactPairFilter` to easily (if maybe not efficiently)
disable collisions between pairs of colliders.

The same could be accomplished by using the `user_data: u128` field of the
`Collider`, but it would be less ergonomic, and also with this PR that
`user_data` space can be saved for more important things.

src/geometry/narrow_phase.rs

@@ -424,6 +424,8 @@ impl NarrowPhase {
                 let context = PairFilterContext {
                     rigid_body1: rb1,
                     rigid_body2: rb2,
+                    collider_handle1: handle1,
+                    collider_handle2: handle2,
                     collider1: co1,
                     collider2: co2,
                 };
@@ -492,6 +494,8 @@ impl NarrowPhase {
                 let context = PairFilterContext {
                     rigid_body1: rb1,
                     rigid_body2: rb2,
+                    collider_handle1: pair.pair.collider1,
+                    collider_handle2: pair.pair.collider2,
                     collider1: co1,
                     collider2: co2,
                 };