[DRAFT] GDExtension: Add compatibility system for virtual methods

[dsnopek]

This is the start of a compatibility system that will allow us to change the signature of virtual methods, and not crash existing GDExtensions that implemented the old version of the virtual methods.

It allows registering compatibility virtual methods, like:

#ifndef DISABLE_DEPRECATED
	// The "_save_compat_100447" is an alias that allows us to refer to this older version of the virtual method.
	GDVIRTUAL3R_COMPAT(_save_compat_100447, Error, _save, Ref<Resource>, String, uint32_t)
#endif

	// This is the current version that has the new signature with the new argument.
	GDVIRTUAL4R(Error, _save, Ref<Resource>, String, uint32_t, FileAccess::SaveIntegrityLevel)

Then when we're calling the virtual method, we can do something like this:

	Error err = ERR_METHOD_NOT_FOUND;

	if (GDVIRTUAL_CALL(_save, p_resource, p_path, p_flags, p_integrity_level, err)) {
		// We return here if the GDExtension implements the current version of the virtual method.
		return err;
	}

#ifndef DISABLE_DEPRECATED
	// If not, we fall back on the old one. Notice the "_save_compat_100447" alias we're using here instead of "_save".
	GDVIRTUAL_CALL(_save_compat_100447, p_resource, p_path, p_flags, err);
#endif // DISABLE_DEPRECATED

	return err;
}

This also changes the extension_api.json file to include a hash for the signature of each virtual method, and updates the GDExtension interface to include a hash argument when Godot is asking the GDExtension for a particular virtual method.

The idea is: the bindings will hang on to the hashes for all the virtual methods they support, and when Godot asks for a particular virtual method, if the hash doesn't match what the GDExtension has, then return nullptr as if the GDExtension didn't implement the virtual method at all. So, if a GDExtension was compiled with an older hash, then the first GDVIRTUAL_CALL() will return false (indicating that the virtual method isn't implemented), but then the second one will succeed because the hash will match what the GDExtension has.

PR godotengine/godot-cpp#1676 are the companion changes for godot-cpp

Marking as DRAFT because there's still a couple things to do:

• Make a godot-cpp PR that implements this
• Do lots and lots more testing :-)
• Store the hashes for the compatibility methods, so we can include them in the JSON
• Compare the hashes for virtual methods when comparing JSON files, so that we can detected missing compatibility methods via CI

NOTE: This presently includes the changes from PR #100447 so that I'd have a change to test, since there is a virtual method signature that we want to change there.

[Zylann]

GDVIRTUAL also includes calls to scripts, so is this going to benefit scripts as well?
Also, will extensions be able to define compatibility virtual methods too?