sampler: Add missing gpu_resource_id() getter

According to the upstream docs and the Metal Shader Converter
implementation the `SamplerState` type has a way to get access to its
`MTLResourceID` on the GPU:
https://developer.apple.com/documentation/metal/mtlsamplerstate/3974101-gpuresourceid

src/sampler.rs

@@ -158,4 +158,8 @@ impl SamplerStateRef {
             crate::nsstring_as_str(label)
         }
     }
+
+    pub fn gpu_resource_id(&self) -> MTLResourceID {
+        unsafe { msg_send![self, gpuResourceID] }
+    }
 }