Add texture_external to inspector

Adds texture_external to the inspector, allowing us to recognize the
type and return provide binding information. Includes a basic test.

Bug: Dawn:728
Change-Id: Ib0f39998359dc22a530ad222141229f9ba30552f
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/51161
Reviewed-by: Ryan Harrison <rharrison@chromium.org>
Commit-Queue: Brandon Jones <brandon1.jones@intel.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

samples/main.cc

@@ -337,6 +337,8 @@ std::string ResourceTypeToString(
       return "WriteOnlyStorageTexture";
     case tint::inspector::ResourceBinding::ResourceType::kDepthTexture:
       return "DepthTexture";
+    case tint::inspector::ResourceBinding::ResourceType::kExternalTexture:
+      return "ExternalTexture";
   }
 
   return "Unknown";