Prevent AnimationGraph from serializing AssetIds.

[andriyDev]

Objective

• A step towards "Always Strong" Asset Handles / Remove asset weak handles #19024.
• AnimationGraph can serialize raw AssetIds. However for normal handles, this is a runtime ID. This means it is unlikely that the AssetId will correspond to the same asset after deserializing - effectively breaking the graph.

Solution

• Stop allowing AssetId to be serialized by AnimationGraph. Serializing a handle with no path is now an error.
• Add MigrationSerializedAnimationClip. This is an untagged enum for serde, meaning that it will take the first variant that deserializes. So it will first try the "modern" version, then it will fallback to the legacy version.
• Add some logging/error messages to explain what users should do.

Note: one limitation here is that this removes the ability to serialize and deserialize UUIDs. In theory, someone could be using this to have a "default" animation. If someone inserts an empty AnimationClip into the Handle::default(), this might produce a T-pose. It might also do nothing though. Unclear! I think this is worth the risk for simplicity as it seems unlikely that people are sticking UUIDs in here (or that you want a default animation in any AnimationGraph).

Testing

• Ran cargo r --example animation_graph -- --save on main, then ran cargo r --example animation_graph on this PR. The PR was able to load the old data (after Bump ron version to 0.10. #19631).

[andriyDev]

Fixed a migration issue with #19631 so this is ready to review!