Expose scene unique id functionality in Resource

[Ryan-000]

Describe the project you are working on

I am making a Multiset plugin for collaborative scene editing, and I need to keep track and update resources over the network across different Godot editor instances.

Describe the problem or limitation you are having in your project

There is no current way to set the embedded resource ID before or when saving a resource. Also, the only way to get the ID is after saving the scene (id generated during save and is based on time!) and splitting the resource path to get the ID.

Not being able to set the scene_unique_id in Resource's is a massive road blocker when creating and keeping track of resources over the network.

When the host creates a new embedded resource in a scene, I cannot keep track of it at all across clients, because the scene unique id is created at save time, so for each client the resource will actually have a different scene unique id, or not one at all. This makes it practically impossible to keep all the changes synced, and will mess up git history, as everyone's will be slightly different.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Expose scene unique id functionality in Resource

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

I made a pull request godotengine/godot#88111

If this enhancement will not be used often, can it be worked around with a few lines of script?

Cannot be worked around its core.

Is there a reason why this should be core and not an add-on in the asset library?

Its Core.