[godot] Fix Skeleton/Texture loader for runtime loading

[TokageItLab]

The API load_from_xxx() must be valid for files outside res:// in the Godot.

• Expose load_from_file() in SpineSkeletonFileResource
• Fix GodotSpineTextureLoader for runtime loading to avoid using ResourceLoader if it is not in the res://
  • load_from_atlas_file() has been fixed to allow loading files outside of res://

Known issues:

1. If there is an image in res://, a load error occurs while the .import is not created, because ResourceLoader is used. If you generate images from Godot and Spine loads them, it is recommended to output them in the user:// folder.
2. There is a problem that Resource.duplicate() does not work for SpineSkeletonFileResource and SpineAtlasResource because Spine's data resources have incomplete serialization integration with Godot's Resource. This PR allows us to serialize the internal data of those SpineResources to our own Resource, and when we want to load Spine, we can temporarily output those serialized data as a file in the user:// folder, and load Spine from there to make it work, although it's bit hacky.

[badlogic]

This is great. The serialization issue is unfortunate, but I don't see a cleaner way around.

I'm sorry for the red tape, but could you digitall sign our contributor license agreement? I need this so we can publish your PR under our Spine Runtimes license. Signing it digitally and sending it to contact@esotericsoftware.com is enough!

This also fixes #2477

[TokageItLab]

I've contacted the esotericsoftware by e-mail previously, and got the format and sent the text. Or do I need a separate electronic seal?

[badlogic]

Thanks!