[DRAFT] 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 https://github.com/godotengine/godot/pull/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 https://github.com/godotengine/godot/pull/113697 is merged, I've updated it to match the latest in Godot master, and taken it out of DRAFT