Allow implementing ScriptInstance::validate_property() from GDExtension

[dsnopek]

Fixes #81119

I haven't had a chance to test this yet.

It adds this function pointer type:

typedef GDExtensionBool (*GDExtensionScriptInstanceValidateProperty)(GDExtensionScriptInstanceDataPtr p_instance, GDExtensionPropertyInfo *p_property);

I opted to have it return GDExtensionBool (the ScriptInstance::validate_property() method doesn't return anything) since we have to copy back and forth between the Godot and GDExtension version of PropertyInfo. I figure this could allow the GDExtension to check if the script even implements validate_property(), and if it doesn't, just return false in order to skip one of the copies. Does that make sense? I'd love some feedback on that.

[YuriSizov]

This PR only allows scripting languages created by extensions to implement this method, so I assume that implementing an extension hook to Object::validate_property itself is a separate issue?

[dsnopek]

Ah, yes, I was focused on scripts because that's what the issue was about, but that would also need to be done. I've just added that to my TODO list as well :-)

[YuriSizov]

The core part of this seems correct to me. If the Object hook is to be done in a separate PR, then this should be okay, I think.

[akien-mga]

Thanks!