Remove unnecessary friend class declarations of CowData.

[Ivorforce]

This is a small cleanup I did while doing sanity checks for #100619.
The friend class declarations are unnecessary. They should be removed because they increase coupling. It additionally makes no sense to have friend classes because CowData should be designed such that it can be used from any class, and not just some known specific classes.

There are only a few affected spots:

_ref() calls can be replaced with operator=(const CowData &) calls. The resulting call is the exact same:

godot/core/templates/cowdata.h

Line 172 in 89001f9

void operator=(const CowData<T> &p_from) { _ref(p_from); }

Forward declarations can also be removed. Two headers were passively using them before (visual_shader and vector). I moved the forward declarations to the appropriate spots.

Finally, I also made _get_alloc_size and _get_alloc_size_checked static. This is a small change that doesn't warrant a separate PR. It does not make sense to have the functions be member functions, because it is relied upon that allocation sizes are consistent across different instances of CowData. It has to be deterministic by class alone.