feat(storage): add diagnostic about deprecated storage gRPC option. (googleapis#15144)

* feat(storage): add diagnostic about deprecated storage gRPC option.

* Formatting fixes

Author: v-pratap

cmake/GoogleCloudCppFeatures.cmake

@@ -512,6 +512,15 @@ function (google_cloud_cpp_define_dependent_legacy_feature_options)
         OR (experimental-storage_grpc IN_LIST GOOGLE_CLOUD_CPP_ENABLE))
         set(GOOGLE_CLOUD_CPP_STORAGE_ENABLE_GRPC_DEFAULT ON)
     endif ()
+
+    # Emit a warning if the deprecated option is explicitly set
+    if (GOOGLE_CLOUD_CPP_STORAGE_ENABLE_GRPC)
+        message(
+            WARNING
+                "GOOGLE_CLOUD_CPP_STORAGE_ENABLE_GRPC is deprecated. Please use -DGOOGLE_CLOUD_CPP_ENABLE=storage_grpc instead."
+        )
+    endif ()
+
     option(
         GOOGLE_CLOUD_CPP_STORAGE_ENABLE_GRPC
         "Enable compilation for the GCS gRPC plugin (EXPERIMENTAL).  Deprecated, prefer GOOGLE_CLOUD_CPP_ENABLE."