[Impeller] added debug info to frame debuggers like AGI (flutter/engine#42717)

fixes https://github.com/flutter/flutter/issues/128629

Screenshot of the change in action:
<img width="740" alt="Screenshot 2023-06-09 at 4 30 48 PM"
src="https://github.com/flutter/engine/assets/30870216/3923c992-323b-466c-bab7-18097c6d1ee2">


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engine/src/flutter/impeller/renderer/backend/vulkan/capabilities_vk.cc

@@ -138,12 +138,13 @@ CapabilitiesVK::GetRequiredInstanceExtensions() const {
     }
     required.push_back("VK_EXT_debug_utils");
 
-    if (!HasExtension("VK_EXT_validation_features")) {
-      VALIDATION_LOG << "Requested validations but could not find the "
+    if (HasExtension("VK_EXT_validation_features")) {
+      // It's valid to not have `VK_EXT_validation_features` available.  That's
+      // the case when using AGI as a frame debugger.
+      FML_DLOG(INFO) << "Requested validations but could not find the "
                         "VK_EXT_validation_features extension.";
-      return std::nullopt;
+      required.push_back("VK_EXT_validation_features");
     }
-    required.push_back("VK_EXT_validation_features");
   }
 
   return required;