Generate GDExtension interface header and loader from JSON

[dsnopek]

This is the first step towards "godot-cpp v10" as described in this proposal.

It does a bunch of things:

• Generates the gdextension_interface.h from gdextension_interface.json
• Replaces most of the code that loads the GDExtension interface functions with code also generated from gdextension_interface.json
• Adds a deprecated=no option for SCons to prevent loading deprecated GDExtension interface functions
• Puts the loaded functions under godot::gdextension_interface rather than godot::internal, making them officially part of the API
• Allows working with both Godot 4.5 or 4.6, depending on which version generated the extension_api.json
• Standardizes on using fully qualified namespaces for ::godot::gdextension_interface and ::godot::internal

There's one important thing this it does NOT do, which is cmake support for the build system changes. That's something I've still got to figure out.

Marking as DRAFT because it now depends on Godot PR godotengine/godot#113697

[dsnopek]

Hrm. MSVC is having issues with using interface as a namespace, but only when building with CMake? That's odd...

[dsnopek]

I ended up renaming interface to gdextension_interface, which I think I like better anyway :-)

[dsnopek]

Now that PR godotengine/godot#113697 is merged, I've updated it to match the latest in Godot master, and taken it out of DRAFT