Add bindings for Metal

[ruccho]

I've experimentally created bindings for Metal(IUnityGraphicsMetal.h).

I'd be happy if we could merge this, but some design issues exist:

• bindgen generates bindings depending on objc for Objective-C headers but I've extracted only pure type definitions from the generated bindings because I would like to use objc2 instead, which is a successor of objc. This operation must be done by hand.

  • header-translator, a binding generator of objc2, seems not to support third party headers: Linking against private frameworks? madsmtm/objc2#501
  • There is a plan to make bindgen to support objc2: Merge header-translator into bindgen madsmtm/objc2#729

• I've implemented high-level bindings on top of objc2 into unity_native_plugin::metal::objc2, which is a module conditionally enabled by the feature objc2_metal, but is it better to move them to an individual crate such as unity_native_plugin_metal like unity_native_plugin_vulkan?