Fix compilation errors and warnings when running bevy_gltf with no features.

[andriyDev]

Objective

• Add support for arbitrary/third party glTF Extension processing via GltfExtensionHandler #22106 accidentally added imports for AnimationClip, HashMap, and HashSet - but these are only available/used if the bevy_animation feature is enabled. So running cargo t -p bevy_gltf fails to compile and gives warnings!

Solution

• Guard these use statements on the bevy_animation feature.

Testing

• Ran cargo t -p bevy_gltf.
• Ran cargo t -p bevy_gltf --all-features.

[andriyDev]

CI failure is an unrelated flake.