sampler: Add missing gpu_resource_id() getter and set_label() setter

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

And if other Metal types in this crate are to be believed, the label is
settable to be able to assign an identifier to it in the first place.

src/sampler.rs

@@ -158,4 +158,15 @@ impl SamplerStateRef {
             crate::nsstring_as_str(label)
         }
     }
+
+    pub fn set_label(&self, label: &str) {
+        unsafe {
+            let nslabel = crate::nsstring_from_str(label);
+            let () = msg_send![self, setLabel: nslabel];
+        }
+    }
+
+    pub fn gpu_resource_id(&self) -> MTLResourceID {
+        unsafe { msg_send![self, gpuResourceID] }
+    }
 }