Expose several resource/resource-saver functions

[Naros]

Exposes several methods for GDExtension and script languages related to Resource, ResourceUID, and ResourceSaver.

Bugsquad edit: The methods are Resource get_id_for_path, is_built_in, reset_state, set_id_for_path, set_path_cache, and ResourceSaver get_resource_id_for_path.

[akien-mga]

There's one issue to fix, but I'd like to see more justification for why these methods should be exposed.

There's no linked proposal, nor any explanation about what you need those methods for.

[Naros]

Hi @akien-mga, exposing a number of these Resource-specific methods is to support custom resource format loaders and savers. Without these, implementors of custom loader/savers have to write their own ResourceCache implementation and this is sometimes at odds with the existing API that use the ResourceCache internally in other Resource methods.

The goal of exposing these is so that if a GDExtension implements a custom resource with custom loader/saver format impls that those implementations can reuse the existing ResourceCache and other facilities within the engine and only focus on supplying the bare implementation needed for the custom resource, loader, and saver wrappers.

In short, this allows a GDExtension implementation of a custom resource loader/saver to be able to work similarly to C++ modules that were to implement the loader/saver interface.

[Naros]

@akien-mga applied your suggestions.

[akien-mga]

Thanks!