Send SceneInstanceReady when spawning any kind of scene

[daxpedda]

Objective

• Emit an event regardless of scene type (Scene and DynamicScene).
• Also send the InstanceId along.

Follow-up to #11002.
Fixes #2218.

Solution

• Send SceneInstanceReady regardless of scene type.
• Make SceneInstanceReady::parent Optional.
• Add SceneInstanceReady::id.

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Changelog

Changed

• SceneInstanceReady is now sent for Scene as well. SceneInstanceReady::parent is an Option and SceneInstanceReady::id, an InstanceId, is added to identify the corresponding Scene.

Migration Guide

• SceneInstanceReady { parent: Entity } is now SceneInstanceReady { id: InstanceId, parent: Option<Entity> }.

[ethereumdegen]

This seems like a nice way to handle this 🙌

It doesnt seem like it would break the ways i use the event. I am now using this event to detect when a GLTF scene is fully attached to an entity [with a specific component] . i loop through the child nodes and act upon them by name. Such as registering the child to an entity linker , swapping the materials, or replacing the child mesh with a xpbd collider. This functionality is critical to multiple parts of my game. It effectively lets me name nodes in blender to give them super powers in bevy. This change seems like it will not break that flow while also addressing the issue 2218

[torsteingrindvik]

Code reads well- the tests really help build confidence here.