Expose `Resource.reset_state()` virtual function

[Jordyfel]

Describe the project you are working on

Godot

Describe the problem or limitation you are having in your project

Came up while investigating godotengine/godot#89026.

I see no reason it couldn't happen for user resources as well.

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

virtual void reset_state(); //for resources that use variable amount of properties, either via _validate_property or _get_property_list, this function needs to be implemented to correctly clear state

The other functions related to fake properties are exposed, so this should be as well.

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

N/A

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

No

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

N/A

[Jordyfel]

completed by #90969