Update gdextension_interface.json and extension_api.json for Godot 4.7-beta1

[dsnopek]

This PR aims to update the GDExtension interface and API files for Godot 4.7-beta1, and make some small build system changes, so that we still default to using the latest stable Godot version (4.6)

This is a DRAFT because Godot 4.7-beta1 still isn't out yet!

But this let's us get ready a bit, and let's me make some follow-up PRs that use the new stuff :-)

[dsnopek]

Taking this out of DRAFT now that -beta1 is released!

[Naros]

@dsnopek I assume that when 4.7 goes stable, extension_api.json is renamed to extension_api-4.6.json, and the 4.7 equivalent is renamed to extension_api.json?

I wonder if it would make more sense to avoid all the shuffling around of extension_api.json if they were always committed with their version and we let default_api_version and api_version dictate which version is used, so that when 4.7 goes stable, it's just a simple default_api_version swap?

[dsnopek]

I assume that when 4.7 goes stable, extension_api.json is renamed to extension_api-4.6.json, and the 4.7 equivalent is renamed to extension_api.json?

Yes

I wonder if it would make more sense to avoid all the shuffling around of extension_api.json if they were always committed with their version and we let default_api_version and api_version dictate which version is used, so that when 4.7 goes stable, it's just a simple default_api_version swap?

That would be easier to maintain, for sure!

However, I went the way I went, because I know there's people whose workflow (either locally or in CI) is to replace the gdextension/extension_api.json file and I wanted that to still work. I think we used to have that in our documentation, actually, which is how it spread initially? The docs now recommend using custom_api_file