Fix const qualifier for parameters in GDExtension api functions

[touilleMan]

see the related Godot-CPP PR: godotengine/godot-cpp#897

The GDExtension contains code like this:

typedef void *FooPtr;
void do_something(const FooPtr foo);

What we'd like to have is foo being typed as const void* (i.e. a pointer on a constant structure), however what it is actually typed is void *const (i.e. a constant pointer on a modifiable structure)

This PR fix this by introducing const types, e.g.:

typedef void *FooPtr;
typedef const void*ConstFooPtr;
void do_something(ConstFooPtr foo);

[touilleMan]

rebased ;-)

[groud]

Approved in GDExtension PR review.

The modification in linux_builds.yml needs to be removed once the related PR in godot-cpp is merged.

[akien-mga]

Thanks!